Compatible numbers are pairs of numbers that are easy to add, subtract, multiply, or divide mentally. While it is hard to mentally calculate 55.59 divided by 7.8, it is easier to mentally calculate 56 divided by 7, because they are compatible numbers, and they are close to the numbers given in the problem (55.59 and 7.8). 56 divided by 7 leaves us with a quotient of 8, which is a good estimate for the quotient of 55.59 and 7.8.
