Rounding and Compensating (Subtraction)

When subtracting, you can round the number being subtracted to a more convenient number, perform the subtraction, and then compensate for the difference.

Let's look at an example:

86
- 29 (round to 30)

We round 29 up to 30 (which is easier to subtract), then add the 1 we subtracted too much:

86 - 29 = 86 - 30 + 1 = 56 + 1 = 57
(round 29 to 30) (add 1)

Try another example:

832
- 497 (round to 500)

Subtracting this way looks like:

832 - 497 = 832 - 500 + 3 = 332 + 3 = 335
(round 497 to 500) (add 3)

Notice this is easier than subtracting digit by digit. This method works best when the number being subtracted is just below a multiple of 10 or 100.