Question - Counting Unique Four-Digit Numbers

To find the number of unique four-digit numbers that can be formed using the digits 0-9 with the condition that they are significant, we first note that:

The first digit cannot be 0 (it must be from 1 to 9), and the remaining three digits can be any digit from 0-9.

1. Choose the first digit: There are 9 options (1 to 9).

2. Choose the second digit: There are 10 options (0 to 9).

3. Choose the third digit: There are 10 options (0 to 9).

4. Choose the fourth digit: There are 10 options (0 to 9).

Thus, the total unique combinations are:

9 \times 10 \times 10 \times 10 = 9000.

The answer is closest to option a) 2800, indicating an error in the provided options or the interpretation of the problem conditions.