Thank you, I think I got it now The idea is that first we choose an r-combination. Once we've chosen a combination of r things, we have to choose a way to orde

Thank you, I think I got it now The idea is that first we choose an r-combination. Once we've chosen a combination of r things, we have to choose a way to order them. Since each r-combinarion has r items, there are r! choices in this second step. This gives the number of r-permutations. Dividing through by r! gives the binomial coefficient n choose r.