You are given the digits 7,2,5,9, and you can only use each digit once in each number.
How many 3-digit even numbers can you form them?
To form a 3-digit even number, the ones digit must be one of 2 or 6. Once we choose the ones digit, we have 4 digits left to choose for the hundreds digit and then 3 digits left for the tens digit. Therefore, the number of 3-digit even numbers we can form is:
$2\times4\times3=24$
So we can form 24 3-digit even numbers.