How many positive integers less than 25 can be formed by using the digits 1,2,and 3?
You'll have to find out whether you can use them more than once.
If you can, then there are these 9
1, 2, 3, 11, 12, 13, 21, 22, 23
If you can't, then there are only these 7
1, 2, 3, 12, 13, 21, 23
Edwin