Rounding and Compensating (Addition)

When adding numbers, you can round one of them to a more convenient number, perform the addition, and then subtract the difference.

Let's look at an example:

78
+ 27 (round to 30)

We round 27 up to 30 (which is easier to add), then subtract the 3 we added:

78 + 27   =   78 + 30 − 3   =   108 − 3   =   105
(round 27 to 30)    (subtract the 3)

Try another example:

739
+ 497 (round to 500)

Adding this way looks like:

739 + 497   =   739 + 500 − 3   =   1239 - 3   =   1236
(round 497 to 500)    (subtract 3)

Notice this is easier then if we were to add (400 + 90 + 7) like we would using left to right addition. So the method works best when one of the numbers is just below a multiple of 10 or 100.