A locker code is a string of 4 digits from 0 to 9. Digits may repeat, but the code must not begin with 0. How many codes are possible?

Explanation

1. 01

   First digit: 9 choices (1 to 9).

2. 02

   Each of the other three digits: 10 choices, since repetition is allowed.

3. 03

   Codes $= 9 \times 10 \times 10 \times 10 = 9000$.