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Abacus - Wikipedia
How to Use an Abacus (with Pictures) - wikiHow
Abacus | Definition, History, Origin, & Facts | Britannica
Abacus Explained — Everything You Need To Know - History Tools
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Abacus Definition & Meaning - Merriam-Webster
Abacus Explained — Everything You Need To Know
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Abacus-World's first calculator | Complete guide to Abacus
Abacus | Definition, Examples, Understanding, Parts, Types & Uses