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Re: crack it
1st digit can be any of 1 to 9 (can't be 0 ,since then the number wouldn't be a 9-digit number. Assuming of course that we're looking to form actual numbers and not just a 9-digit code)
2nd digit can be any of 9: 1 less than 0 to 9 to account for the 1st digit
3rd digit can be any of 8, 4th can be any of 7 etc..
Total number of 9-digit numbers that have different digits that exist = 9*9!
Too lazy to calculate.
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