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Using the digits 0-9, how many six digit numbers can be formed if no digits can be repeated?
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1. Any of the digits from 1 to 9 in the leftmost position. 9 options.
2. Any of remaining 9 digits in the next to the right position. 9 options.
3. Any of remaining 8 digits in the next to the right position. 8 options.
4. Any of remaining 7 digits . . . 7 options.
. . . . And so on . . .
6. Any of remaining 5 digits in the rightmost, 6-th position. 5 options.
In all, there are 9*(9*8*7*6*5) = 136080 such numbers.