In the third grade, compatible numbers are estimated figures that are used to make mental arithmetic simpler. Compatible numbers are very close to the actual numbers and yield a similar result with less work.
When determining compatible numbers, a student must consider what numbers divide cleanly into other numbers. For example, 5 and 10 both divide evenly into any number that ends in a 0. All even numbers divide neatly into other even numbers. All of this must be considered when picking compatible numbers.
For example, if a student needs to divide 9 into 70, he can round the 9 up to 10 so that it will divide evenly into 70. The answer with the actual numbers is 7.778, while the answer with compatible numbers is 7. If the problem is 31 divided by 4, 31 can be rounded to 30 and 4 can be rounded to 5. The answer with actual numbers is 7.75, and the answer with compatible numbers is 6.