Question

Which expression represents the number of possible permutations of n items taken r at a time?
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Answer 1

The number of permutations of n items taken r at a time is
P(n,r) = n! / (n − r)!,
equivalently n·(n−1)·(n−2)·…·(n−r+1) (for 0 ≤ r ≤ n).