Find number of four-digit numbers in which repetition is not allowed.

Explanation

To find the number of four-digit numbers in which repetition of digits is not allowed, we can follow these steps:

### Step 1: Determine the choices for the first digit
- The first digit of a four-digit number cannot be 0 (as it would then be a three-digit number). Therefore, the possible choices for the first digit are from 1 to 9.
- This gives us **9 options** for the first digit.

### Step 2: Determine the choices for the second digit
- The second digit can be any digit from 0 to 9, except for the digit already used in the first position.
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