b) How many zip codes are possible if the first digit must be a 9 ? Solution:

Step 1 ：The problem is asking for the number of possible zip codes if the first digit must be a 9. A zip code in the United States is a five-digit number. Since the first digit is already determined to be 9, we only need to find the number of possibilities for the remaining four digits.

Step 2 ：Each digit can be any number from 0 to 9, which gives us 10 possibilities for each digit.

Step 3 ：Since there are 4 digits left, the total number of possibilities is \(10^4\).

Step 4 ：Final Answer: There are \(\boxed{10000}\) possible zip codes if the first digit must be a 9.