There is only one possible combination as a combination is the same no matter what the order is. There are 120 permutations, which is where the order does matter.
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6,720 combinations.
10x9x9x9
There are 2 possible digits for the first digit (3 or 4), leaving 3 possible digits for the second digit (5 and 6 and whichever was not chosen for the first), leaving 2 possible digits for the third. Thus there are 2 × 3 × 2 = 12 possible 3 digit numbers.
If repetition of digits isn't allowed, then no13-digit sequencescan be formed from only 5 digits.
There are two possible digits for the first and last digit, and two possible digits for the centre digit, making 2 × 2 = 4 possible 3 digit palindromes from the set {1, 2}, namely the set {111, 121, 212, 222}.