Starting with three different letters, six two-letter combinations can be made, if the order of the two letters is important. Only three combinations can be made if the order of the two letters is not important.
Example: ABC
AB AC BA BC CA CB - six variations
But if (for your purposes) BA is the same as AB,
Then there are only three:
AB AC BC
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24
6
it is 26
Four combinations: tam ham mat hat Hope it helps (:!
If the numbers can be repeated and the numbers are 0-9 then there are 1000 different combinations.