What does compatible number mean in math?

Answer 1

Compatible number is one that is near the actual number, but is easier to use for addition, subtraction, multiplication, or division. In other words, it is easy to use for estimating the answer to an arithmetic problem.

Example:

38 + 23

A compatible number for 38 might be 40

A compatible number for 23 might be 20 (or 25)

The sum of the compatible numbers would be 60 (or 65). So you know the answer will be near there.

If you choose 40 and 20, you know that one of your compatible numbers is a little too high, and the other is a little too low. So your estimate of the answer (40+20=60) will be pretty close. (Your estimators are high by 2 and low by 3, so you know your answer is low by 1. It is really 61, not 60.)

If you choose 40 and 25, you know that both of your compatible numbers are a little too high, so your answer will be a little more high. As it turns out, both "estimates" are high by 2, so the estimated answer (65) is high by 2+2=4. The actual result is 61 = 65-4