10^9
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If every number can be used as many times as you like, there are 104 = 10000 different combinations. If each number can only be used once, there are 9!/(9 - 4)! = 5040 combinations.
9!/5! or 3024
There are 9C2 = 9*8/(2*1) = 36 2-digit combinations.
There are 10 choices for the first number, 9 for the second and 8 for the third. 10*9*8=720 possible combinations.
9