Compatible numbers are simple or simply added numbers with close values to more difficult numbers, substituted in their place for ease of calculation. An example would be substituting 15 and 3 when trying to multiply 14.8 and 2.6.

Numbers are considered simpler when they are round, whole or even. This means quicker processing and more effective mental math. When using compatible numbers, North Star Math recommends finding whole or simple numbers with the least deviation from the original number. Finally, while they are a useful measure in situations where more exact sums are not possible, using compatible numbers reduces precision.