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.

 

 

Subtraction Trick 1

Source: https://www.facebook.com/australianteachers/photos/a.1409177146069885.1073741828.1409169412737325/1505873516400247/?type=3&theater

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.

 

Subtraction Trick 2

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1 Comment

  1. Yeah, that does make the problem a lot easier, doesn’t it?

    Reply

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