"Courtney plans to display four of nine stamps in a special frame. In how many orders can she display the stamps?"
I'm know it's $9!$ to show the different ways that nine stamps can be shown (because you have $9$ options for the first stamp, and then $8$ for the next, and $7$ for the next, etc.) but I'm not sure how to only have four of those stamps.
It's equal to $\binom{9}{4}\cdot4!=\frac{9!}{4!5!}4!=\frac{9!}{5!}=9\cdot8\cdot7\cdot6=3024$, since there are $\binom{9}{4}$ ways to choose $4$ out of $9$ stamps and then $4!$ ways to order the $4$ chosen stamps.