7
20
The answer is 10C4 = 10*9*8*7/(4*3*2*1) = 210 ways.
32C3 = 4960
24
3 x 3 x 3 x 3 = 81 combinations
20
The answer is 10C4 = 10*9*8*7/(4*3*2*1) = 210 ways.
120 combinations using each digit once per combination. There are 625 combinations if you can repeat the digits.
32C3 = 4960
24
3 x 3 x 3 x 3 = 81 combinations
15
two: 36 and 63
It is: 9C7 = 36
4x4x4x4=256
There are 59049 combinations, all the way from 11111, 11112 through 99998, 99999. There are too many to list here.
137, 173, 317, 371, 713, 731 Six combinations can be made.