## Thinking Mathematically (6th Edition)

The order of the chosen shirts does not matter. Combinations, ${}_{n}C_{r}=\displaystyle \frac{n!}{(n-r)!r!}$ ${}_{4}C_{2}=\displaystyle \frac{4!}{(4-2)!2!}=\frac{4\times 3}{1\times 2}=$ 6 ways