## Precalculus (6th Edition) Blitzer

We know that the order in which the three CD’s are selected does not make a difference since all would be taken for the vacation at the same time. Since the order of the arrangement does not make a difference, we solve the problem with combinations.. The three CD’s are to be selected from a group of twenty CD’s. So, $n=20,r=3$. Hence, \begin{align} & _{20}{{C}_{3}}=\frac{20!}{3!\left( 20-3 \right)!} \\ & =\frac{20!}{3!17!} \\ & =\frac{20\times 19\times 18\times 17!}{6\times 17!} \\ & =1140 \end{align}