1,000. The list looks just like the counting numbers from 000 to 999 .
120 combinations using each digit once per combination. There are 625 combinations if you can repeat the digits.
There are 9999 possible combinations starting from 0000 to 9999
9
There are 15, and they are:1234 1235 1236 1245 1246 1256 1345 1346 1356 1456 2345 2346 2356 2456 3456.
Not repeating, it is 7*6*5*4 which is 840 ways ---- There are 7 choices for each of four digits, right? 74 = 2401
Number of 7 digit combinations out of the 10 one-digit numbers = 120.
It is: 9C7 = 36
66
10,000
15
Through the magic of perms and coms the answer is 729
There are 840 4-digit combinations without repeating any digit in the combinations.
If there are no restrictions on duplicated numbers or other patterns of numbers then there are 10 ways of selecting the first digit and also 10 ways of selecting the second digit. The number of combinations is therefore 10 x 10 = 100.
There are 360 of them.
120 combinations using each digit once per combination. There are 625 combinations if you can repeat the digits.
Each digit can appear in each of the 4 positions. There are 9 digits, therefore there are 9⁴ = 6561 such combinations.
9