Arithmetic Grade Three

Lesson Thirty-nine: Compensation

Compensation

Compensation is a method you can use to help you do addition or subtraction in your head.

In this example we have 85 + 38. It's a lot easier to add 85 + 40 and so we transform the problem by adding 2 to 38. When we add 85 and 40 we get 125. So, to compensate for the two we added earlier, we subtract 2 from the answer and we get a final answer of 123.

Here's another example:

Here we have 55 - 17. It's a lot easier to do 55 - 20 in our head and so we add 3 to 17 to get 20. The answer to 55 - 20 is 35. Since we took away an extra 3 in the altered problem, we must add 3 to the answer which gets us 38.

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