## Precalculus: Concepts Through Functions, A Unit Circle Approach to Trigonometry (3rd Edition)

There are 9 selections for the first choice ( because 0 is excluded) and 10 selections each for the second, third and fourth choices. Using the fundamental principle of counting, the number of 4-digit numbers that can be formed is: $= 9\times10\times10\times10 \\= 9000$