Number of 7 digit combinations out of the 10 one-digit numbers = 120.
There are 840 4-digit combinations without repeating any digit in the combinations.
9^9 = 387420489 different digit combinations.
Through the magic of perms and coms the answer is 729
There are 360 of them.
1,000. The list looks just like the counting numbers from 000 to 999 .
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.
120 combinations using each digit once per combination. There are 625 combinations if you can repeat the digits.