They are used particularly for quicker mental calculation.
They are close to the original numbers, but are nice, round numbers, and can be conveniently used for calculation.
For instance, in a calculation involving 48 and 18, we can choose compatible numbers 50 and 20 for convenience.
This methodology can be used to estimate different results, even in problems involving percentages and fractions.
To simplify the concept, simply remember that this estimation is a form of rounding.
All you need to do is round off these numbers to the nearest whole numbers and find the solution quickly.
Read more at Buzzle: http://www.buzzle.com/articles/using-compatible-numbers-for-quick-and-easy-arithmetic-calculations.html