Dealing with permutations and arrangements again.The number of possible permutations of r members of a set containing n elements is n! / (n–r)!. A permutation is an ordered arrangement of objects from a set containing at least as many members as in the permutation. An unordered set is a combination.
The number of possible combinations of r members of a set containing n elements is n! / ( (n – r)! * r!).