## Precalculus (6th Edition) Blitzer

We know that according to the fundamental counting principle, 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. There are a total of 10 digits $0$ to $9$, to make a five-digit zip code. The first digit can be filled by digit $1$ to $9$; therefore, there are 9 choices to fill the first digit. Thus, in sequence, the second digit can be filled by 10 choices; the third, fourth and fifth digits can be filled by ten choices $0$ to $9$ because $0$ cannot be filled in the first digit. According to the fundamental counting principle, the total number of ways to make a five-digit zip code is $9\times 10\times 10\times 10\times 10\times 10=900000$