For example, the permutations of the set {a,b,c} are 3!=6: abc, bac ,bca, cab, cba, acb.

The number of k-combinations (each of size k) from a set S with n elements (size n) is the binomial coefficient:

where n is the number of objects from which you can choose and k is the number to be chosen.

For example, the combinations of the set {a,b,c,d} with 2 elements are 4!/(2!2!)=6: ab, ac, ad, bc, bd, cd.