Last time we talked about combinations and permutations with groups of different items, or more formally put sets of n items each of which was unique. This time I'm going to explore what happens if the groups we're working with aren't all different items. A classic example of this being phone numbers where a number can reoccur a number of times in some cases.

If you want to look at combinations where not all of the set are different then the formula that you use is

**n**C

**n1, n2**