Abacus is an ancient calculator originated from China. It was used for basic mathematic operations and solve questions like square roots. There are certain rules and formulae to follow when using an abacus. When a user can operate an abacus proficiently, not only will he calculate the answer very quickly, but it will also imprint the abacus’s shape and its beads into the brain by stimulating brain cells, almost like having an invisible abacus. This imprinted image of the abacus is called “phantom abacus”. Calculating mentally on this phantom abacus, is what we call “mental arithmetic”.
Once a person become adept in mental arithmetic, he can easily outpace an electronic calculator. Often, he can immediately produce an answer when he hears or sees the problem.