The number of different four digit numbers that can be formed with the digits 2, 3, 4, 7 and using each digit exactly once,

Since each digit is used exactly once, for four digit number, _ _ _ _

First place has 4 options:

Second place has 3 options left

Third place has 2 options left

and Last place has 1 option

Therefore, number of different four digit numbers positive

= 4 × 3 × 2 × 1

= 24