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Listed under:  Mathematics  >  Number (Mathematics)  >  Number operations  >  Associativity
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Chinese abacus, c1900

This is a rectangular wooden abacus (15.5 cm x 29 cm x 2.5 cm), made in about 1900. It has two decks (divided horizontally by the beam) of 13 rods inserted vertically in the frame. On the bottom deck there are five wooden beads on each rod and on the top deck there are two beads on each rod. The abacus has a removable wooden ...

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Wishball: hundreds

Test your understanding of place value with three-digit numbers. Start with a three-digit whole number such as 507. A spinner provides a randomly generated digit. Choose its place value and add it to (or subtract it from) your starting number. Work towards a given target number, say 539, using other digits. You can choose ...