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

- combination: nCr = number of selections of "r" items taken from a set of
"n" distinct items (order does NOT matter!!)
= $\frac{{n!}}{{\left( {n - r} \right)!\;\;r!}}$

Permutation vs. Combination

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