Addition and Subtraction with Two Fingers

I noticed my second grader was counting on his fingers to solve simple arithmetic. Somehow memorizing the answer to 17-9 wasn’t worth it. He was perfectly happy employing 8 fingers in pursuit of a solution. Now most kids can add and subtract easy numbers ending in zero or five. So it occurred to me that if you are going to count on your fingers, you only need two fingers at most to change the problem into something easier. For example,

17
-9
—-
8

Can be transformed to an easy problem by adding one to both the minuend and subtrahend

18
-10

8

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