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
nC n1, n2