There are ten combinations: one each where one of the ten digits, 0-9, is excluded.
There are 59049 combinations, all the way from 11111, 11112 through 99998, 99999. There are too many to list here.
Through the magic of perms and coms the answer is 729
There are 360 of them.
Each digit can appear in each of the 4 positions. There are 9 digits, therefore there are 9⁴ = 6561 such combinations.
100
It is: 9C7 = 36
10,000
15
Number of 7 digit combinations out of the 10 one-digit numbers = 120.
There are 59049 combinations, all the way from 11111, 11112 through 99998, 99999. There are too many to list here.
Through the magic of perms and coms the answer is 729
216
There are 840 4-digit combinations without repeating any digit in the combinations.
There are 5,040 combinations.
120 combinations using each digit once per combination. There are 625 combinations if you can repeat the digits.
There are 210 4 digit combinations and 5040 different 4 digit codes.
There are 360 of them.