The answer is 10C3 = 10*9*8/(3*2*1) = 120 combinations.
6
Only one.
10
Six combinations: 123, 132, 213, 231, 312, 321
120 combinations using each digit once per combination. There are 625 combinations if you can repeat the digits.
If repeats are allowed than an infinite number of combinations is possible.
10,000.
45
If the 6 digits can be repeated, there are 1296 different combinations. If you cannot repeat digits in the combination there are 360 different combinations. * * * * * No. That is the number of PERMUTATIONS, not COMBINATIONS. If you have 6 different digits, you can make only 15 4-digit combinations from them.
5040, assuming none of the digits are the same. (Assuming they're not, there's 5040 unique combinations you can make out of 7 digits).
6
45 In combinations, the order of the digits does not matter so that 12 and 21 are considered the same.
The number 1234567890 can create about 39 of 40 4 digit numbers.
Only one.
Assuming no duplication or repetition, 720
Assuming the digits cannot be repeated, there are 7 combinations with 1 digit, 21 combinations with 2 digits, 35 combinations with 3 digits, 35 combinations with 4 digits, 21 combinations with 5 digits, 7 combinations with 6 digits and 1 combinations with 7 digits. That makes a total of 2^7 - 1 = 127: too many for me to list. If digits can be repeated, there are infinitely many combinations.
10