Postal Service The U.S. Postal Service uses 5-digit ZIP codes to route letters and packages to their destinations.
a. How many ZIP codes are possible if the numbers 0 through 9 are used for each of the 5 digits?
b. Suppose that when the first digit is 0, the second, third, and fourth digits cannot be 0. How many 5-digit ZIP codes are possible if the first digit is 0?
c. In 1983, the U.S. Postal Service introduced the ZIP 4, which added 4 more digits to the existing 5-digit ZIP codes. Using the numbers 0 through 9, how many additional ZIP codes were possible?