9x8x7
because the first digit has 9 possibilities (1-9), the second digit has 8 possibilities (1-9 minus the one already used), and the 3rd has 7 possibilities (1-9 minus the 2 already used)
So there are 504 numbers like that
500
It is 120 if the digits cannot be repeated.
4*3*2*1 = 24
6*5*4*3*2*1=720
That would be 4! = 4 * 3 * 2 * 1 = 24
24
500
There are 7,290 different 4-digit numbers that can be formed from the digits 1-9 without repetition.
There are 5*4*3 = 60 such numbers.
1 set
It is 120 if the digits cannot be repeated.
27 if the digits can be repeated. 6 if not.
3x3x3=27
36
6*6*3 = 108 numbers.
4*3*2*1 = 24
6*5*4*3*2*1=720 possible numbers