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There are 9 digits that can be the first digit (1-9); for each of these there is 1 digit that can be the second digit (6); for each of these there are 10 digits that can be the third digit (0-9); for each of these there are 10 digits that can be the fourth digit (0-9). → number of numbers is 9 × 1 × 10 × 10 = 900 such numbers.
There are 90 two-digit numbers... starting with 10 and ending with 99.
There are 9*10*9 = 810 such numbers.
720 (10*9*8)
30,25,15,20, or 10.