6 ways: 931,913,139,193,391,319
Only three: 12, 13 and 23. Remember that the combinations 12 and 21 are the same.
9
120 combinations using each digit once per combination. There are 625 combinations if you can repeat the digits.
Six combinations if you use each digit only once per combination: 157 175 517 571 715 and 751.
10 Combinations (if order doesn't matter). 3,628,800 Possiblilities (if order matters).
If you can repeat the numbers, it will be 6*6*6=216. If you can't, it will be 6*5*4=120.
It is: 9C7 = 36
66
15
10,000
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 720 combinations if you use each of the digits only once per combination.