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References

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Socher, K. (2004-2018). A short explanation on how to use the Chinese abacus (suanpan). Retrieved from http://tuxgraphics.org/scienceclub/abacusexplained.shtml

 

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What is a ten key? (2018, March 12). Retrieved from http://www.wisegeek.org/what-is-a-ten-key.htm#comments

 

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