## Thinking Mathematically (6th Edition)

The number of ways in which a series of successive things can occur is found by multiplying the number of ways in which each thing can occur. ------------ You would want to see in how many ways you can choose the 1st digit of the ZIP code, the 2nd digit, the 3rd digit, the 4th digit, the 5th digit, and, since this is a series of successive choices, you would apply the FTC. (the answer to each successive question is 10, and the total is $10^{5}=10,000$ zip codes ) Makes sense.