6*5*4*3*2*1=720 possible numbers
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
5040 different 4 digit numbers can be formed with the digits 123456789. This is assuming that no digits are repeated with each combination.
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