Abacus-World's first calculator | History of Computers
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What is an abacus? The history of abacus calculating device. | Kурсове по английски, БЕЛ, математика и ментална аритметика
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How to Calculate Using an Abacus
How Do You Calculate on the Abacus? | blog@CACM | Communications of the ACM
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How to Use an Abacus (with Pictures) - wikiHow
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