Mental strategies for addition and subraction

Compensation in addition and subtraction?

You can often convert an addition or subtraction question into an easier question by temporarily adding or subtracting an appropriate small number. For example, many people would evaluate 673 + 99 by adding 1 temporarily to the 99, so the question becomes 673 + 100. This gives 773. Now take off the extra 1, to get the answer 772. This strategy is sometimes called compensation.

The trick in the strategy is always to be on the lookout for an easier calculation than the one that you have to do. This will often involve temporarily replacing a number ending in a 9 or an 8 with the next multiple of 10.

For example, the subtraction, 83 – 28, can temporarily be changed into the much easier calculation, 83 – 30, which gives 53, with 2 to be added to this to compensate for the fact that we’ve actually taken away 2 more than required.