# A nice subtraction trick

A friend of mine recently posted this trick for subtracting any number from any multiple of $10^n$. (I discovered this trick when I was a boy and have been using it ever since.)

Pedagogically, I don’t think I’d recommend requiring every elementary school student to learn this trick. But this does make a nice enrichment activity for talented elementary school students, as it requires conceptual understanding of subtraction and not just the ability to follow a procedure.

Here’s another approach, taken from the comments of the above webpage: consider 5000 as 500 groups of 10 and 0 groups of 1, and then regroup.

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