People have counted, added and subtracted with an abacus since ancient times. The name comes from the Greek word, abax, meaning "board" or "calculating table", used for doing or teaching arithmetic." Arithmetic includes addition, subtraction, multiplication and division. The abacus can also be used to work with fractions and with finding square and cube roots.
The advantages of its use over pencil or pen arithmetic is time . It is faster than writing down the problem and solving it. When working with the abacus one just needs to be accurate. You leave the memory work to the abacus frame.
For more info, check the folllowing link
http://www.geocities.com/learner_center/abacus.html 
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