Chapter 12.3, Problem 41E

In Exercises 31–44, specify the number of ways to perform the task described. Give your answers using $\text{P}\left(\text{n, r}\right)\text{ or C}\left(\text{n, r}\right)$ notation. The key in recognizing whether a problem involves permutations or combinations is deciding whether order is important.

Garage door opener codes. In order to open your garage door, you must enter a four-digit code on a keypad with numbers 0–9. Each number may not be used more than once, and the order that you enter the numbers is important.