I should have mentionned it, but I rather think about it first. Suppose f:A->B and g:B->C then g*f:A->C That makes sense. Then they talked about f*g! Is this possible? Think about it. If f has domain A, and g has range C, than how can f(x) equal anything, since the range of C is not equal to set A, which is what we need to calculate f. f*g:B->? I hope you understand.