Exercise 10 Page 841

Many country codes are $3\text{-}$digit numbers. We want to find the number of all possible different $3\text{-}$digit country codes. To do so, we need to first evaluate how many different choices there are for each digit. Let's list the numbers we can choose from. There are $10$ single-digit numbers to pick from. We know there are no restrictions regarding second and third digits of the code, so they can be chosen from all $10$ numbers. However, there is a restriction on the first digit. Country codes do not begin with a $0$ or $1.$