Since f is the number of female cousins and the number of male cousins is 4 less than the number of female cousins, we can write the following expression to represent the number of male cousins:
For example, if the number of female cousins is 7, we can use this value (f = 7) in the expression above to find the number of male cousins:
So if the number of female cousins is 7, the number of male cousins is 3.
Therefore the expression for the number of boy cousins is f - 4.