Question

How many 4-digit numbers (numbers with 4 digits) can you make with the leftmost digit is 1 without repeating any digit?

502

503

504

505

Answer 1

504

Explanation:

The left most number must be 1

The second digit can be anything but 1. There are nine numbers to choose from.

The third digit can be anything but 1 and the second digit. There are 8 to choose from

The last digit can by anything but 1, the second digit and the 3rd digit. There are 7 to choose from.

1 * 9 * 8 * 7

504