well, just start listing them:
7539 -> 1
78.. -> 4P2
79.. -> 4P2
8... -> 4P3
9... -> 4P3
now add 'em up.
How many 4 digit numbers are greater then 7538 can be made using the digits, 0,3,5,7,8,9 without repetition?
3 answers
So the answer would be 73?
works for me.