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(Permutations formula)
P(n,r) = n!/(n-r)! is the number of ways to arrange r objects chosen from n.
Why this formula? n! counts all arrangements of n. Divide by (n-r)! to cancel the unused positions.
Example: P(5,3) = 5!/(5-3)! = 5!/2! = 120/2 = 60. Matches our previous count.
Special case: P(n,n) = n!/0! = n!/1 = n!.
