what is a compatible number in subtraction?

Question

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Answer 1

Pick the compatible number for the number which will make the subtraction easier to carry out in your head. For example, 78-59. Use 60 instead of 59 and you get 18 as the answer. Or use 58 instead of 59 so that the 8's cancel out to give you 20. The actual answer is 19 which is close to 18 and 20.

You could also replace 78 by 79 so that the 9's cancel out, giving you 20.

You could also replace both numbers with 80 and 60, giving you 20.