Subtraction can be checked by addition, and addition can be checked by subtraction:
To check 82 - 37 = 45, you could add 45 and 37 and see if you get 82.
To check 56 + 31 = 87, you could take 31 away from 87 and see if you get 56.
Division can always be checked by multiplication, and multiplication can be checked by division:
To check 81 ÷ 3 = 27, you could multiply 27 by 3 and see if you get 81.
To check 9 x 12 = 108, you could divide 108 by 9 and see if you get 12.
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