An abacus is a manual aid to calculating that consists of beads or disks that can be moved up and down on a series of sticks or strings within a usually wooden frame. The abacus itself doesn't calculate; it's simply a device for helping a human being to calculate by remembering what has been counted.
In this Lesson 3 we explained how can do Addition and Subtraction using 10's complement on the Abacus.
Ages: As soon as a child learns to count on their fingers they are ready to start using the abacus. However, ages 8 and up will benefit the most. Older kids will already be comfortable with basic math skills and will feel that these videos move a bit slow, but can still benefit significantly by learning the abacus.