22.5 Permutations


Imagine, you want to withdraw cash from the ATM machine, but you forget the password of your account. The only thing you remember is that the password consists of the number 1, 2, 3, and 4. You are now trying to figure out the number of ways to arrange the numbers, and that’s permutations. Unlike combinations, order matters in permutations!


? permutation: nPr = number of arrangements of r items taken from a set of n distinct items (order matters!!)
= n!(nr)!\frac{{n!}}{{(n - r)!}}
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