Question

Using all the one digit prime numbers, each only once, how many four digit even numbers can be formed?

Answer 1

The only one-digit even prime number is 2, so that number is the last digit. There are 3 choices for the first digit. There are 2 choices for the second digit. There is 1 choice for the third digit. So 3 × 2 × 1 = 6 four-digit even numbers can be formed.