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Number 3 is arleady placed on theabacus (fig
1). Now, number 4 is to be added. The three hypobeads in the first column are already used, the two hypobeads left are not enough for four. So, move down one altobead (fig 2) and take away one hypobead (fig 3). What we did was to add 5 and to subtract 1, which is the same as to add 4 (5-1 = 4). |
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| fig 1 | fig 2 | fig 3 |
