3x3x3=27
500
24
5040 different 4 digit numbers can be formed with the digits 123456789. This is assuming that no digits are repeated with each combination.
1 set
It is 120 if the digits cannot be repeated.
27 three digit numbers from the digits 3, 5, 7 including repetitions.
There are 5! = 120 such numbers.
4*3*2*1 = 24
48 of them if digits may not be repeated. 100 if they can.
6*6*3 = 108 numbers.
36
6*5*4*3*2*1=720 possible numbers