## Thinking Mathematically (6th Edition)

$r!$
Comparing ${}_{n}P_{r}=\displaystyle \frac{n!}{(n-r)!}\quad$ and $\quad {}_{n}C_{r}=\displaystyle \frac{n!}{(n-r)!r!}$, the extra factor in the denominator of ${}_{n}C_{r}$ is $r!$