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10-94.

Here is another way to think about the question: “What is $0!$?”

1. How many ways are there to choose all five items from a group of five items? What happens when you substitute into the factorial formula to compute $_{5}C_{5}$? Since you know (logically) what the result has to be, use this to explain what $0!$ must be equal to.

$_{n}\textit{C}_{r}=\frac{\textit{n}!}{\textit{r}!(\textit{n}-\textit{r}!)}$

2. On the other hand, how many ways are there to choose nothing from a group of five items? And what happens when you try to use the factorial formula to compute $_{5}C_{0}$?

See part (a).