9!/(5! x 4!) = (9 x 8 x 7 x 6) / (4 x 3 x 2) =126 if all digits have to be different.
If repeated digits are allowed then it is 95 ie 59049.
10,000
36
99999
5040 different 4 digit numbers can be formed with the digits 123456789. This is assuming that no digits are repeated with each combination.
If the digits can be used more than once, then 900. If not, then 648.
500
24
840
There are 60480 numbers.
180 of them.
There are 3024 of them.
1 set