According to Math Goodies, compatible numbers in math are whole numbers close in value to the real numbers but allow easier computation. Compatible numbers are either rounded up or down to give a whole number. The answer from compatible numbers should be close in value to the correct figure.
Compatible numbers are only used in mental computation. Working with decimals can make it difficult to multiply and divide, so compatible numbers help to simplify these tasks. For example, with the equation 5.8 multiplied by 3.2, the compatible numbers would be 6 and 3. The sum of these compatible numbers is 18, while the actual answer is 18.56, which is close enough to the sum of the compatible numbers when only an estimation is needed.