Permutation
A permutation of a set of r objects is any rearrangement of the r objects. There are 
(the factorial of r) permutations of a set with r objects.
An important question is how many ways to pick an r-element subset of a set with n elements, where order matters. To find how many ways we can do this, note that for the first of the r elements, we have n different objects we can choose from. For the second element, there are (n-1) objects we can choose, (n-2) for the third, and so on. In general, the number of ways to permute r objects from a set of n is given by
.
See also
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