Three Finger Tricks for Multiplying also made me think of another finger trick: using both hands to count to 99. It’s not as clever as multiplication, but it’s one I use regularly for keeping track of numbers and for simple addition or subtraction.
The idea is simple: the left hand keeps track of tens while the right hand keeps track of units. On the left hand, the thumb is worth 50 and each of the other fingers is worth 10. Likewise, on the right hand, the thumb is worth 5 while each of the other fingers is worth 1. This picture (below) shows 27:
while this picture (below) shows 67:
I first learned this in 2nd grade, when it appeared as an article in my Weekly Reader magazine. The article mentioned that this method was used by children in Asia (as it mimics an abacus) and that it could even be modified for addition, subtraction, and multiplication. Addition and subtraction seem straightforward, in the sense that the hands can be used to keep a running tally, but I have yet to figure out how multiplication would be done. [Indeed, having taught multiplication on an abacus, I feel like I would need to grow several more hands before I could apply that technique].
Still, for keeping track of 2-digits numbers, it’s simple and definitely memorable!