6*6*3 = 108 numbers.
Assuming no repeated digits, lowest first, 20; in any order 120; Allowing repeated digits: 216
1 set
48 of them if digits may not be repeated. 100 if they can.
There are 5! = 120 such numbers.
6*6*3 = 108 numbers.
192 ways
Assuming no repeated digits, lowest first, 20; in any order 120; Allowing repeated digits: 216
24
500
There are 5*4*3 = 60 such numbers.
1 set
It is 120 if the digits cannot be repeated.
There are 7,290 different 4-digit numbers that can be formed from the digits 1-9 without repetition.
3x3x3=27
36
4*3*2*1 = 24