Cardinality of Set of Subsets/Proof 2

Proof
Let $\dbinom n m$ denote the number of subsets of $m$ elements of $S$.

From Number of Permutations, the number of $m$-permutations of $S$ is:
 * ${}^m P_n = \dfrac {n!} {\paren {n - m}!}$

Consider the way ${}^m P_n$ can be calculated.

First one makes the selection of which $m$ elements of $S$ are to be arranged.

This number is $\dbinom n m$.

Then for each selection, the number of different arrangements of these is $m!$, from Number of Permutations.

So: