When Is Cheryl's Birthday? Answer To Viral Math Puzzle

Рет қаралды 387,281

9 жыл бұрын

This problem went viral after it appeared on a Math Olympiad test for talented students in Singapore. It's not a trick question! Here's the problem, which I have very slightly re-worded for clarity.
Blog post (detailed text explanation): mindyourdecisions.com/blog/201...
Source: kennethjianw...
Official Solution: mothership.sg/2015/04/p5-logic...
Albert and Bernard have just become friends with Cheryl, and they want to know when her birthday is. Cheryl gives them a list of 10 possible dates.
May 15, May 16, May 19
June 17, June 18
July 14, July 16
August 14, August 15, August 17
Cheryl tells Albert the month and Bernard the day.
Albert says, "I don't know when Cheryl's birthday is, but I know that Bernard cannot know either."
Bernard then says, "At first I didn't know when Cheryl's birthday is, but now I do know."
Albert concludes, "Now I know when Cheryl's birthday is."
So when is Cheryl's birthday?
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Пікірлер: 733

@erikaz1590 2 жыл бұрын

You forgot to mention that both Albert and Bernard are perfect logictians! That's like the biggest clue in all of these I swear.

@quarkhead5044 2 жыл бұрын

After hearing the word 'logician' in a problem, you just know that you HAVE to read the problems many more times.

@taki.nam3 Жыл бұрын

logictians??

@dannyluo2077 8 ай бұрын

@@taki.nam3logician

@zakenco 7 жыл бұрын

This can be solved much more easily without doing math, Albert and Bernard work together to find Cheryl's birthday by exchanging info.

@kitchencone 7 жыл бұрын

Okay, Cheryl is annoying. If you don't want us to know when your birthday is, just say so. Oh, and I have 60 watermelons left over from another math problem that I need to get rid of.

@andrewu8525 7 жыл бұрын

What math problem, or are you annoying like Cheryl not give vague riddles to people

@kitchencone 7 жыл бұрын

Sorry, I thought everyone knew that one. Just Google 60 watermelons.

@ayg2282 7 жыл бұрын

kitchencone maths is the only place where a person buys 10273539294 watermelons and no one questions why

@devendra_c 7 жыл бұрын

kitchencone Hahaha....!!

@skyla2258 6 жыл бұрын

Hahahahahahahahahahaha

@Averia89 7 жыл бұрын

Wait, why go through all this confusion? If Cheryl told Albert the month and Bernard the day, then wouldn't they just ask each other what she said?

@bekichhakchhuak8226 4 жыл бұрын

They really should just do that and let us know.

@crystalexia- 4 жыл бұрын

logic 100

@fos1451 3 жыл бұрын

Jarrel where’s the fun in that?

@HDitzzDH 3 жыл бұрын

Maybe she offered a price to the one who could figure it out, if they helped each other then presumably neither would get a price. Or maybe they just really hate each other lol.

@jck3195 3 жыл бұрын

Coz cheryl is a bitch🤣🤣

@rhandeymaahrsch2151 3 жыл бұрын

Cheryl: I bet you can’t figure out when my birthday is. Albert: This hammer and your kneecap disagrees.

@aritragupta4182 7 жыл бұрын

A better way to explain it would be - Choices with unique days fall in May (May 19) and June (June 18). Now Albert knows for certain Bernard does NOT know Cheryl's birthdate. The only way Albert could be certain is if he knows that the day is not unique. The only way he could be certain of that is he were told a month none of whose dates are unique. That eliminates May and June. Now consider Bernard saying 'I know now'. Remember he knows the day AND he's eliminated May and June. Now if he were told '14' he could NOT be certain since 14 occurs in both July and August. So July 14 and August 14 are eliminated. We're now left with 1 day in July and 2 days in August. And we come to the final clue which is when Albert claims he knows the birthdate. Now see Albert knows only the month. If it were August he would not be able to choise between August 15 and August 17. The only way he can know for sure is if he knows the month to be July because July 14 being eliminated earlier there is only one choice left - July 16

@ibyzrulez 7 жыл бұрын

Aritra Gupta Finally a great explanation, was just a little stuck on the elimination of May and June

@ANDREW-tq3ut 5 жыл бұрын

Thank you, sir! You're really good! I wasn't able to understand quickly while watching the video so I went to the comments and read yours, and finally, I got to understand it fully! Thank you, once again!

@saratoapanta2492 5 жыл бұрын

Ooooh! I get it now :( thanks!!

@gsanks 4 жыл бұрын

Excellent explanation - much better than the video

@derekdukes1010 3 жыл бұрын

Why was June 17th eliminated? I understand May 19 and June 18. When Bernard states I, he now knows. The 17th is the only day that doesn't repeat.

@MindYourDecisions 9 жыл бұрын

A lot of people are wondering why all the May and June dates are eliminated from Albert's first statement. Think about it from Albert’s perspective. If he heard May, he would not be sure whether Bernard might have heard 19. Or if he heard June, he would not be sure whether Bernard may have heard 18. But if he heard July or August, he can be sure that Bernard would hear a day that's ambiguous. So logically he must have heard July or August, and that's why he said he knows for sure Bernard does not know. And Bernard, a mater logician, would realize this too. You may not like this interpretation, but it is the only logically consistent explanation. For those still in doubt, you can check the intent from the official answer key: mothership.sg/2015/04/p5-logic-question-is-actually-a-math-olympiad-question-for-sec-3-and-4-students/

@gustavb3673 9 жыл бұрын

MindYourDecisions It all adds up to how Albert know that Bernard doesn't know. If he was told so by Cheryl then it's a fact but if it's an assumption then the logic changes.

@KiterTMK 9 жыл бұрын

MindYourDecisions ***** You are wrong, and you should demonstrate this by reenacting the interaction between Albert and Bernard for this explenation to work properly After eliminating the unique 18 and 19 dates you still have 15 and 16 double, and you have 2 months of May. If it were 15, Bernard still wouldn't know if it would have been May 15th or August 15th. If it were 16, Bernard still wouldn't know if it's May 16 or July 16th. And if it were May, Albert still wouldn't know whether it's May 15 or May 16. They cannot rule out May 15 or 16th. But they do know that June 17 was left. If It were June, there is no other choice besides 17 and he would've told he knew. It isn't June, because Albert still doesn't know whether it's 1 of the 2 or 3 dates of May, July or August. Albert knew it isn't 18 or 19 because he knew that Bernard would have known from the start. Now that July 17th is eliminated, Bernard knows, because the date 17 became unique and there is no other choice besides August 17th. Albert follows the logic that he can only know if there is a unique number left and vice versa for Bernard. Bernard didn't know at first, because after eliminating 18 and 19, it could have been either June or August 17th. After Albert indicated that it isn't the remaining date of June, Bernard could tell with certainty it's August 17th.

@HumptyDumptyOakland 9 жыл бұрын

KiterTMK All the dates in May are ruled out if Albert has been told any month other than May. If Albert has been told May he can't be sure that Bertrand hasn't been told 19 (unless he's been secretly told that Bertrand doesn't have 19, which is a totally unreasonable assumption to make) When Albert makes his 1st statement Bertrand can deduce that Albert doesn't have May, Jul16 is the only answer that satisfies Albert's 2nd statement.

@ownagebutter 9 жыл бұрын

MindYourDecisions the way you rule out may and june doesn't make sense. The reason albert can say " i don't know but i know bernard doesn't know either" is because if bernard was given a date with 19 or 18, he would have spoken up right away. This would still leave two ambiguous dates in may

@CloudDinoGod 9 жыл бұрын

MindYourDecisions the whole point of the question was analyzing the statements....so abvoiiusly if u dont understand, its a hard question.........dont just listen to what he says, understand it

@ou2m7DNSKxrulHPIU6I86f 4 жыл бұрын

Presh, please give more videos on puzzle like this that only requires pure logic to solve without needing to know any theorem, it's so fun!!!!!!

@WayneJohnsonZastil 9 жыл бұрын

Albert says "I know it's July" Bernard says "I know it's 16" Both say "Wow that was easy July 16th"

@shivastate 9 жыл бұрын

The bigger question is "what is the color of cheryl's dress?"

@bebaoboy 5 жыл бұрын

And is Cherry's Cheryl favorite fruit till now? 💤

@Lemonbread123 4 жыл бұрын

It's yanny

@jayasrim8963 4 жыл бұрын

No what us the number of hairs on his/her head.

@galangz575 4 жыл бұрын

@@jayasrim8963 69420

@richardlloyd2589 3 жыл бұрын

Dress? Well you lack ambition! 😉

@deadeaded 9 жыл бұрын

To those confused: this is a logic puzzle. When A says "B doesn't know" that means "Using *only* information about the month, we can deduce that it is impossible for B to know". It does *not* mean "B hasn't given an answer yet".

@nikkyu8264 8 жыл бұрын

I really like this video. Good work MindYourDecisions. Awesome stuff

@Artist_Sir 2 жыл бұрын

Obviously, May 19 and june 18 go off by Albert's sentence. But why does entire month of May and June go off? Can you help me?

@HavidzNurrizqi 9 жыл бұрын

maybe this is the full conversation: Cheryl: Albert, the month is ... and Bernard, the day is .... Albert: "I don't know when Cheryl's birthday is, but I know that Bernard cannot know either." Bernard: Wtf? why are you so sure? if I got May 19 or June 18, I know the answer man. Oh I see, if you got May or June, you'll tell me 'maybe you know the answer' and can't be sure like that. Well, now I can see my number, "At first I didn't know when Cheryl's birthday is, but now I do know." Albert: What? if you have 14, you must be confused, so if I remove them, July 16 is the one left. "Now I know when Cheryl's birthday is."

@adriandraganescu6453 6 жыл бұрын

This is the best explanation so far! Thanks dude

@radhikamulukutla9124 4 жыл бұрын

Very well explained, I understood now after reading this, thanks

@veronikavalentovicova4398 4 жыл бұрын

Omg I still dont understand it!! I'm trying so hard but nah... I just dont get it how you guys exclude all may and june if me (let's say I'm Albert) having the month july, Bernard can have 14, or 16. Either way he cannot know my month, because if he has 14, then I can gave july or august, and if he has 16, then i can have may or july. So how does Bernard know which month do I have??

@anasahmad4788 4 жыл бұрын

you are gold

@MangOnang611 4 жыл бұрын

@@veronikavalentovicova4398 Bernard will know that it was July 16 because May 16 was eliminated due to Albert's statement. If the number is 14, Bernard still couldn't know because there's July 14 and August 14. I guess the important part is to include that after Cheryl told Albert the month and Bernard the date, nobody guessed. Seeing this condition, Albert then said the 1st statement. If Bernard had a unique number like 18 or 19, he could guess as soon as Cheryl told him the number. But he didn't, thus Albert's comment.

@datsmydab-minecraft-and-mo5666 5 жыл бұрын

It took like 30 mins to figure out in the very first sentence is what it took me the longest because at first I thought to remove only may 19 and June 18 then came across the idea that it’s because 18 and 19 are the only days Bernard could say the birthday from and Albert knows that Bernard didn’t get may 19 or June 18 because the month that Cheryl gave Albert wasn’t nor June or May (hopefully I cleared some things up)

@pgoughms 5 жыл бұрын

It doesn't make sense. How does Albert know that Bernard doesn't know it?

@dannyluo2077 2 жыл бұрын

Albert was told July, and the days 14 and 16 aren’t unique.

@sergiolunaamaya7443 9 жыл бұрын

The explanation is consistent. Thanks.

@Fidder492 2 жыл бұрын

A variation of this puzzle was given to me as an assessment exam in a university I was applying to. I'm glad I was able to watch this video a couple years back :)

@benheideveld4617 2 жыл бұрын

An interesting deduction is that the University seems to assume it is possible in the limited amount of time of a formal exam to figure out the whole solution mechanism and then apply it to find the correct answer. In reality they are just selecting viewers of Presh Talwalkar to enter the University. What does that tell you about the quality of the professors at that University?

@hissatsu5703 5 жыл бұрын

I cant believe i got it right

@ryanjagpal123 3 жыл бұрын

Did you guess it?

@Artist_Sir 2 жыл бұрын

Obviously, May 19 and june 18 go off by Albert's sentence. But why does entire month of May and June go off? Can you help me?

@angelisdania1594 2 жыл бұрын

There’s a preliminary bit of necessary logic which, when applied, invalidates the riddle. It must be assumed by Albert before he makes his first statement, that Cherryl would not have given Bernard the days 19 or 18, as that would be the same as telling him the month and day, which would have made it pointless to give one person only the month and the other person only the day. By extrapolation, this also means that Cherryl would not have given Albert the month of June, as knowing Cherryl wouldn’t have given Bernard the days 18 or 19, it would leave only one June option, so it would again be effectively giving Albert both the day and the month before any exchange of words. This means that if Albert is told either May, July, or August, he will not know Cherryl’s birthday, and he will also know that Bernard does not know Cherryl’s birthday. However, that information once stated will not allow Bernard to narrow down to one date, thereby invalidating the riddle.

@Sydrooo 7 жыл бұрын

I got it! Wooooooo! :D I watched your other puzzle similar to this a few days ago and now I know how to logically solve it. Thanks a lot man! :D

@Artist_Sir 2 жыл бұрын

Obviously, May 19 and june 18 go off by Albert's sentence. But why does entire month of May and June go off? Can you help me?

@Sydrooo 2 жыл бұрын

@@Artist_Sir So Albert states in his first sentence that Bernard does not know the birthday either. Assume Bernard knew her birthday was on the 16th, then it could be May 16th or July 16th, and he would not know which one. But assume Bernard knew her birthday was on the 19th, then Bernard WOULD know her birthday straight away as the 19th May is the only date with the number 19. However, Albert says Bernard does not know the birthday, therefore you know the birthday is not in May. This same logic applies for all dates in June. 18th is only an option in June, so if Bernard was told her birthday was on the 18th, he would straight away know 18th June is her birthday. But Albert says Bernard DOES NOT know her birthday based on the information Bernard has, so her birthday is not in June either. I hope this makes sense to you

@Bell_414 3 күн бұрын

I have never seen someone so happy in a youtube comment section. all smiles :D :D

@rand2002 4 жыл бұрын

Today is July 16th, 2020 I remember watching this video a year or two ago. Wonder if it was on July 16th then? I wonder if Presh can use logic to tell my why KZfaq put this video back in my feed today?

@googlee1877 9 жыл бұрын

Wow, this is both interesting and frustrating. I couldn't solve this and even had some trouble understanding the solution. But I finally got it the next day. No wonder the kids from Singapore are smart and a few years ahead.

@DadoMac 9 жыл бұрын

What makes this puzzle so confusing to me is that I am solving a problem with two hypothetical persons in it who are solving the puzzle themselves. As I have to solve two variables (month and day) I tend to forget that Albert and Bernard had only one each, and that they also were given a constant figure. After focusing on these facts, I was able to understand the logic from the deductions.

@ihavenolifebutilikeboobsth6035 8 жыл бұрын

Answer: Cherly's friends are fucking smart as shit!

@humairakhan7263 8 жыл бұрын

truee😂

@QwertQwert-qo3le 7 жыл бұрын

IHaveNoLife ILikeBoobsThough holy shit your right.

@alexanderhoward122 7 жыл бұрын

Damn. I know how it could be formulated better! "Albert cannot know the birthday, having only month name, and he says "I have figured out, having only the name of month, that Bernard cannot know it either having only day number."". It would have been clearer. Or just replace "...but I know Bernard..." with "...but I have figured out that Bernard..." that would have increased the clarity.

@AssasssinConstantine 7 жыл бұрын

Can you please explain the logic, why it can not be May or June

@milanvissers40 8 жыл бұрын

simple way Bernard and Albert actualy say the date and day out loud problem solved

@dxjxc91 7 жыл бұрын

See. THIS is the kind of puzzle I like seeing you talk about. Tricky, but in a purely logical kind of way, such that there is only 1 right answer. And no misleading symbols that you have to infer the meaning of.

@jackfrost9159 9 жыл бұрын

The Answer is July 16th Remember, Albert is told either May, June, July or August. Bernard is told either 14, 15, 16, 17, 18 or 19 Let’s go through it line by line. Albert: I don’t know when Cheryl’s birthday is, but I know that Bernard doesn’t know too. All Albert knows is the month, and every month has more than one possible date, so of course he doesn’t know when her birthday is. The first part of the sentence is redundant. The only way that Bernard could know the date with a single number, however, would be if Cheryl had told him 18 or 19, since of the ten date options only these numbers appear once, as May 19 and June 18. For Albert to know that Bernard does not know, Albert must therefore have been told July or August, since this rules out Bernard being told 18 or 19. Line 2) Bernard: At first I don’t know when Cheryl’s birthday is, but now I know. Bernard has deduced that Albert has either August or July. If he knows the full date, he must have been told 15, 16 or 17, since if he had been told 14 he would be none the wiser about whether the month was August or July. Each of 15, 16 and 17 only refers to one specific month, but 14 could be either month. Line 3) Albert: Then I also know when Cheryl’s birthday is. Albert has therefore deduced that the possible dates are July 16, Aug 15 and Aug 17. For him to now know, he must have been told July. Since if he had been told August, he would not know which date for certain is the birthday. The answer, therefore is July 16.

@tubekec 9 жыл бұрын

Jack frost "For Albert to know that Bernard does not know, Albert must therefore have been told July or August, since this rules out Bernard being told 18 or 19". Please elaborate...how did you eliminate May and June, I understand eliminating May 19 and June 18, that's all...

@billobrien5625 9 жыл бұрын

tubekec Albert was not given enough information to eliminate specific dates. All he knows is the month. The only way he could know for sure that Bernard was not given one of the two unique day was if he had been given a month that did not contain them. If he had been given May or June then the POSSIBILITY would exist that Bernard had been given a unique day and would therefore know the month as well. For him to say for certain that B did not know means he was give July or August and ALL of May and June

@Datta123456 9 жыл бұрын

tubekec Because the three statements in the question is not the full story. We don't know if Cheryl told Albert any more clues or vice versa. All we know is the three statements from their conversation. Therefore we must assume "For Albert to know that Bernard does not know" that Cheryl gave him some more clues. Thus May and June are deducted.

@didac99 9 жыл бұрын

***** Finally, Thanks! xD

@JustTheHighlights 9 жыл бұрын

Sonjoy Datta No you don't need to assume that there were more clues. All the information required to solve the riddle are in the riddle.

@kelvinou4957 5 жыл бұрын

The answer lies in the wording, Bernard “may” know points to the fact that the given date to Albert is May or June Bernard “doesn’t” know points towards the fact that the Months cannot be May or June because he’s so damn sure about it. If A is given May or June, he would have used the sentence above, not this.

@michaelgreen4206 5 жыл бұрын

OK but how does Bernard get to say "I didn't know at first but now I do know"?

@webermaster8595 7 жыл бұрын

I finally start to catch on. After watching a million of these videos I get how logic works now

@farrel_ra 4 жыл бұрын

Ikr! Logic is so fun, thats wut make me interested in it and decide to expand my knowledge of it in computer science! 😊

@Artist_Sir 2 жыл бұрын

Obviously, May 19 and june 18 go off by Albert's sentence. But why does entire month of May and June go off? Can you help me?

@bestredditstories1158 4 жыл бұрын

The answer must be July 16th. Based on the first statement by Albert, it immediately rules out May and June as possible dates since there’s a possibility that, if told those dates, Bernard would know the Birthday immediately but since Albert made it clear that it isn’t possible that Bernard would know when the birthday is, those dates can’t possibly be the birthday. Then, the next statement by Bernard says that after Albert made his statement, Bernard knows the birthday, the answers also cannot be July 14th or August 14th, since if he was told the number “14” he still wouldn’t know. That leaves us with three possible choices. The fact that Albert now knows the birthday after Bernard makes his statement leaves us with the only possible month where a single month and date are still left. The answer is then July 16th.

@literallylegendary6594 3 жыл бұрын

Me 239 days later: *clicks this video, instantly pauses and comments this sentence*

@aditishikha8149 4 жыл бұрын

The question would have become much easier if Albert would have said "Bernard cannot know it" instead of saying "Bernard does not know it" .Think it over!

@tenj00 8 жыл бұрын

Presh Talwalkar. Cool name Sir. Are you by any Chance related to Guybrush Treepwood?

@swaleenelic4950 4 жыл бұрын

before answer reveal: May 19?

@AuddityHipHop 8 жыл бұрын

I GOT IT WOOOT took like 20 minutes also i had to figure to assume albert and brenard had access to the possible dates, which you didnt actually specify in the video. that took a few minutes

@suhailkarim12 6 жыл бұрын

Sounds From The Basement trust me the level of happiness you get after finally getting it

@AuddityHipHop 4 жыл бұрын

4 years later took my 5 minutes this time lol.

@nettefairy 9 жыл бұрын

If Albert was given May or June, then Albert would know there's a chance Bernard might know the answer.

@barttemolder3405 Жыл бұрын

That stumped me at first. Albert can only be certain that Bernard does not know the answer if the month does not contain a date with an unique day, which rules out May and June which do have an unique day. From his answer Bernard can deduce the month is either July or August. Albert knows Bernard knows the date so it can't be the 14 as it appears in both. July has 1 remaining date and August has 2; as he now knows the date as well that needs to be July - the 16th.

@harmidis 9 жыл бұрын

Great logic!

@Artist_Sir 2 жыл бұрын

Obviously, May 19 and june 18 go off by Albert's sentence. But why does entire month of May and June go off? Can you help me?

@oneinabillion654 5 жыл бұрын

How does Albert know beforehand that Barnard doesn't know her birthday?

@legendgames128 3 жыл бұрын

What about the general case for x days and y months and z possibilities?

@wullum8902 2 жыл бұрын

i know i'm nit picking here, but it being clear that albert and bernard are both aware that it is one of the ten possible dates should be in the initial information

@geekygamer3954 5 жыл бұрын

Why is may 16 removed?

@caelidog7645 7 жыл бұрын

+MindYourDecisions, please don't call this a math puzzle. It's a logic puzzle and you know they're not the same thing.

@akhtar74644 5 жыл бұрын

whats the problem in May16??

@kishanthacker4154 3 жыл бұрын

Lets say that bernard was told 17, so in this case after the Albert gave his sentence thr Bernad will know as there is only Aug 17 left with the 17. So in this case also he would have said that now he knew the birthday. The question is that how did Albert got the hint that Bernard had been told 16? In the bottomline I would say that Aug 15 & Aug 17 & July 16 all are equally probable.

@rohannuckchady2900 8 жыл бұрын

oh maths olympiad , i was wondering how that problem made it to a regular exam xD

@wintersprite 3 жыл бұрын

How do we know Cheryl is even telling the truth to Albert and Bernard?

@volodymyrgandzhuk361 2 жыл бұрын

In logic problems, everyone is assumed to be telling the truth unless stated otherwise

@arjonfulgencio1726 5 жыл бұрын

Advanced happy b-day Cheryl

@ziyulu 3 жыл бұрын

I've watched numerous videos and read many web posts, but still am so confused. I still do not see the logic.

@Resedda 2 жыл бұрын

1: If Albert was told May or June, then the possibility for 18 and 19 would be there. Since he knows for sure that it is not 18 or 19, he gives away that he doesnt have May or June. 2: Bernard can not have 14. Or else he would not know the answer. 3: Albert, sitting on July, understands that B does not have 14. The only other option is 16. If Albert was sitting on August, then he could not have stated that he knows the answer.

@stanwagon4570 2 ай бұрын

It is not enough to say that Albert and Bernard are "smart". In problems like this one must also say that Albert knows that Bernard is smart, that Bernard knows that Albert is smart, that Albert knows that Bernard knows that Albert is smart, and, in some problems of this kind, the same ad infinitum. This is (almost) always ignored, and so makes the problem not properly stated from a mathematical point of view.

@bach556 3 жыл бұрын

You have to mention that akbert and bernard know that cheryl’s birthday is one of those 10 possibke dates

@icarus-wings 3 жыл бұрын

Sure, but on what day of the week was she born?

@EnergeticWaves 7 жыл бұрын

I see a flaw in alberts reasoning. Bernard can figure the day because he knows the number and can exclude certain answers however, albert does not know the number and would not know between 15 16 or 17. Please explain.

@Nothing-je4bw 6 жыл бұрын

Albert knows the month

@saraflint2982 6 жыл бұрын

Bernard knows the day of the month is 16. But he doesn't know if it's May or July. That's where Albert's first statement comes from. He was told July. He knows that Bernard was told 14 or 16, which are not unique dates. So he knows that Bernard can't figure out the month. They deduce their answer from each other's statements.

@_tiff.y3275 9 жыл бұрын

is it august 17?

@resomonk6860 9 жыл бұрын

Many people are conflating what we can deduce, knowing neither the month nor the day, and what Albert and Bernard can deduce, knowing them respectively. Say for instance Bernard was the first one to speak. He would say, "I do not know." We would know that it is not May 19 or June 18, because they are unique dates. But Albert, without hearing from Bernard, says he is certain that Bernard does not know, meaning he is certain it is neither of those dates. What information could tell him that, having heard nothing from Bernard upon making the statement? What information was he given? The month. So what Albert is essentially doing is eliminating those months, saying, "I know the month, and it is not May or June."

@HDitzzDH 3 жыл бұрын

But how does he knows it's not May or June? All he knows is that it's not May 19 or June 18 specifically, those two dates are out of the question obviously, but to eliminate the entirety of May and June from just that information seems ridiculous.

@brendanstaff269 7 жыл бұрын

I cant find my tin foil hat

@hector5851 9 жыл бұрын

The answer might depend on interpretation. Alternative 1: If you think that A knows that B doesn't know her birthday because B told A that he doesn't, or from any other source, then, the answer is August 17th. Alternative 2: If you think that A knows that B doesn't know her birthday based entirely on his own knowledge (and what C told him, of course), then, the answer is July 16th. I think that's the source of confusion in this problem. We could say that the second alternative is slightly more correct because it doesn't assume more events, such as B saying A that he doesn't know C's birthday.

@hector5851 9 жыл бұрын

***** I agree. I just wanted to understand and explain why many people were getting August 17th. After all, the problem is stated verbally, not as a formula. That introduces some degree of ambiguity. It's not uncommon in tests for a question to have more than one correct answer. But in a test, I'd definitely go for July 16th. As I said, I wanted to understand the mental process of the guys getting August 17th.

@zukozaider5804 4 жыл бұрын

why can we eliminate May and June, just because he wasn't told the date 19 or 18?

@keithwongchuking5499 9 жыл бұрын

i dont get it why you canceled out the first 10 in the top MindYourDecisions

@brellow-416 6 жыл бұрын

Awesome Presh!

@TomMaynard--TCM-- 9 жыл бұрын

What I had failed to grasp was that the dates *ARE* the answer (or at least guide one to it). Until that moment, I felt it was impossible. Thanks for yet another great video, Presh.

@Deltacadet84 7 жыл бұрын

Isn't June 17th another possible answer by the same logic?

@HumptyDumptyOakland 9 жыл бұрын

This is the correct answer. Some people use "shoddy logic" to arrive at Aug17, but that answer is based on either unsubstantiated assertion or assumption.

@HumptyDumptyOakland 8 жыл бұрын

Shanee Bahera Aug17 is not a correct answer based on the information provided

@mathsific2964 8 жыл бұрын

+HumptyDumptyOakland Agree

@justinlewtp 8 жыл бұрын

+Shanee Bahera The question setters in Singapore rejected Aug 17. So try again

@michaelrussian1086 7 жыл бұрын

you get August 17 when you don't eliminate May 15 and May 16, and July 16 if you do. there's no reason to eliminate May 15 and 16 though

@HumptyDumptyOakland 7 жыл бұрын

+Michael Russian _"there's no reason to eliminate May 15 and 16 though"_ Sure there is, Albert wasn't told May therefore ALL days in May are necessarily eliminated

@Itosalix 3 жыл бұрын

Ok, based on that information, what color was the bear?

@ashishkadakia1129 3 жыл бұрын

How I can solve using programming languages?

@annadoesroblox6205 4 жыл бұрын

It took a little while but I figured it out.

@sarastemkar7835 5 жыл бұрын

Didn't get the 14 July m aug part plz explain

@jessstuart7495 7 жыл бұрын

I don't follow how you eliminate May 15, May 16, and June 17 at 2:00. Bernard is still uncertain about the month on either May 15/August 15, May 16/July 16 or June 17/August 17.

@sisbrawny 9 жыл бұрын

I understand why May 19 and June 18 can be ruled out. But I still don't understand how the rest of May and June can be. How do we know Albert wasn't told the month was May or June? Same for the second step - How do we know Bernard wasn't told the date was 14? How is it possible for him to eliminate July 14 and August 14?? If he was told 14, he wouldn't know... and we don't know what date he was told. It's like we're just _assuming_ he wasn't told 14, which doesn't make any sense.

@BY-to1rc 9 жыл бұрын

sisbrawny Based on Albert saying "I know Bernard doesn't know the date" That means Albert know Bernard wasn't given a month that had a day that was only listed once (18 and 19). That allows Bernard to rule out May and June. For your second questions, Bernard said "At first I didn't know (because he had a repeated day), but now I do". This means the date is one that doesn't have a repeated day of the remaining 5 (allowing you to get rid of July 14 and August 14). If he was told 14, then he would never have said "but I do now".

@SomnoNaut 9 жыл бұрын

Gary White Albert CANNOT know that though.

@SomnoNaut 9 жыл бұрын

Gary White "Albert saying "I know Bernard doesn't know the date"" Is immediately prima facie information after Cheryl whispered to Date ONLY to Bernard. This statement is equally valid Bernard saying "I know Albert doesn't know the month" And what does that or the original offer...nothing.

@BY-to1rc 9 жыл бұрын

***** Of course he can know that. Bernard was given a day of the month. By Albert saying Bernard doesn't know that date means Albert knows the only two dates that had non repeating days was in May and June. If Albert was given any month other then those two then he could make that claim as fact. If he was given either of those months then he couldn't make that statement. That is how you can rule out both of those months.

@billobrien5625 9 жыл бұрын

***** Why would Bernard state: "I know Albert doesn't know the month" when that information was given to him by Cheryl?

@bradarmstrong3952 2 жыл бұрын

Yay! I'm learning from watching these puzzles -- I actually got this one!

@bappabiddu3296 4 жыл бұрын

It's still not clear that why upon assumption the dates (especially first two dates) of May have been eliminated and likewise for the rest. This is not a full proof logic but an assumed logic... Could you please explain this Mathematically?

@medvfx3370 8 жыл бұрын

nowdays he could've just went to her facebook why the fuck would he use his mind anyway?

@DrGlutton1 Жыл бұрын

ChatGPT failed it, it says its 14 of July))) sad(

@gablab89 2 жыл бұрын

I don’t understand something. Why do you eliminate all the dates in a month that has a unique day? For example, if Albert had May, he could infer that Bernard could have May 15 or 16, but not May 19. His statement would still be true since he does not know which day in May it is and Bernard does not know which month goes with 15 or 16. This thought process doesn’t seem to add up.

@limsayjoon165 3 жыл бұрын

How can you rule out May when the dates on May is repeated in June and July. It doesn't make sense to me.

@Resedda 2 жыл бұрын

If Albert has May, then B might have 19. If Albert does NOT have May, then B does not have 19. If Albert has June, then B can have 18. Since Albert has neither May nor June, he can state that Bernard does not have 18 or 19.

@devondevon4366 Жыл бұрын

Albert only needed to say, 'I know Bernard doesn't know," as he didn't have to say he didn't know too since the only way he could have known was if there was only one date with July, and July 14 was not there. On the other hand, Bernard couldn't have known if he was 18 or 19. Hence by Albert saying, " I know Bernard doesn't know," he was actually telling Bernard that he didn't know May or June, but either July or August since there were May 18 and June 19, and the other two months didn't have 18 or 19.

@ravenmitchell820 8 жыл бұрын

I still don't get why it's not May or June.

@shubhamkumbhakar2389 4 жыл бұрын

Happy Birthday Cheryl❤🎂

@Xeverous 7 жыл бұрын

You have aligned month each line but days are only sorted. Day columns overlap and might confuse some people, especially explaining 1st statement

@viviwidynt1196 4 жыл бұрын

Wait, please explain why May 15 and may 16 and June 17 were eliminated ? I dont get it. I understand that May 19 and June 18 were eliminated. Thankyou

@enteryournamehere6093 4 жыл бұрын

If Albert got May/June then he would say "i don't know if Bernard knows the birthday" because of May 19 and June 18. But he said that he DEFINITELY doesn't know, which means he either got July or August.

@TheHarshBaisla 3 жыл бұрын

Wait i dont k ow why u erased may 15 and 16

@yumao3627 5 күн бұрын

Step 1: Albert doesn’t know the date but he knows that Bernard doesn’t either. That means Albert’s month is not unique but all of the day numbers associated with it also associate with at least one other month. Step 2: Bernard deduces the month. That means his day number associate with only one month that doesn’t associate with any unique day numbers

@mohammadabumaash9024 8 жыл бұрын

How did Albert assume that the date Bernard knows was wrong at first?

@HumptyDumptyOakland 8 жыл бұрын

+Mohammad Abu Maash He didn't assume it, he deduced it from the information (i.e the month) he was given

@twoken3266 6 жыл бұрын

1. Albert was told the month, so when he says Bernard wouldn’t know, that means the month could only be July or August because July and August are the only 2 months with matching days with other months, hence the months with 19 and 18 should be crossed out. (Because Albert doesn’t know the date, so the fact he can be so sure is because the months do not contain the days 19th and 18th which if date is 19 or 18 then Bernard would’ve known.) 2. Bernard now knows the month when Albert say that because may and June are eliminated, and the possibility is that the day he got is the only day left among the dates, so 14th could also be eliminated. Now we have July 16, August 15 and aug 17 left. 3. Albert now says he also knows the date! That means the month he knows must have been the single one left so the 2 Augusts are eliminated. Which means the only date left is July 16. 😇

@devondevon4366 Жыл бұрын

First: Could Bernard or anyone have known her birthday if only given the 'day'? Yes, 19 or 18 since May and June are the only two months with unique number19 or 18 (May 18 and June 19), as there are two months with 14, 15, 16, and 17 each. So, if Albert said, "I don't know, but I know Bernard doesn't either," Then it meant he (Albert) was not given June or May; hence her birthday was either in July or August. Since Bernard knew that he had 16, which was either in August or July, BUT there was no August 16, it must be July 16. So, he said, " I now know Cheryl's birthday," which, in turn, gave the answer to Bernard that it was not July 14, but July 16 because if Bernard had a "14", he COULD NOT have found the solution because of August 14 (another month with 14). So, Albert found the solution as well, July 16.

@anikaloves 5 жыл бұрын

my bday is on 16 july..am i cheryl????

@cheatermaster100 9 жыл бұрын

why can't albert tell bernard the month or bernard tell albert the day?

@JustTheHighlights 9 жыл бұрын

If that happened, there would not be a riddle for us.

@danielkhan1 7 жыл бұрын

So they will have an excuse why they saved money to not buy anything for Cheryll

@noahali-origamiandmore2050 7 жыл бұрын

That One Weird kid browsing through the Internet, your explanation makes sense.

@vbvamsi17 4 жыл бұрын

Wait.. I didn't understand the last part it was reduced to July 16 Aug 15 and Aug 17 and Bernard could figure it out.. Then how did Albert figure it out he didnt know what Bernard was thinking..anyway after Albert says I also know now we can figure it out but how did Albert know?

@maybeweare7357 11 ай бұрын

I'm happy to say I solved it in 16 minutes because I watched a lot of your videos ❤

@robertsullivan9833 9 жыл бұрын

why couldn't the date be july 14th? if Bernard got 14 and albert got july then july 14 would work.

@schwarzerritter5724 9 жыл бұрын

1:32 I thought Albert knew Bernard doesn't know the answer, because Bernard hasn't said May 19 or June 18 yet. And from that point the problem was pretty much unsolvable to me. I was able to narrow it down to 6 dates though.

@dannyluo2077 2 жыл бұрын

Albert wasn’t told May or June, the only months with unique days, so that was how he knew Bernard couldn’t know Cheryl’s birthday.

@bhavanasutar3185 5 жыл бұрын

Why did you eliminate the first five? any logic?

@user-dx1rq8gl2n 3 жыл бұрын

Same im still confused

@MisterJasro 9 жыл бұрын

Where I read it the who was told what was not given. So I ended up with an if then for who was told what. If you reverse what Albert and Bernard were told it's a different solution but in that case there is also only one solution. The 17th of June.

@salumtummundi9462 5 жыл бұрын

Wrong

@zebra3stripes 8 жыл бұрын

I think the Empire had an entire fleet of presh talwalkars in one of the prequel movies.

@tootoontanbaku 9 жыл бұрын

good thinking, thanks.

@justinatest9456 9 ай бұрын

It took everything I had, and I could barely explain it back to myself, but I got there, and with the same reasoning. Now I'm spent...

@aimaimaii 5 жыл бұрын

Well i just thought its june 18. It says that the number must be unique so ut must be 18 or 19. Then you can figure it easily by checking the 2nd clue "at first". That means it should be june 18.