Find the number of odd integers between and in which no digit is repeated.
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The numbers we're interested in begin with one of 3 , 4 , 5 , 6 , 7 (we trivially discount 8 0 0 0 ). They end in an odd digit, one of 1 , 3 , 5 , 7 , 9 .
If the first digit is odd , there are 4 choices for the final digit; then 8 choices for the second and 7 for the third digit, for a total of 4 × 8 × 7 = 2 2 4 numbers.
If the first digit is even , there are 5 choices for the final digit; then 8 choices for the second and 7 for the third digit, for a total of 5 × 8 × 7 = 2 8 0 numbers.
Three of the possible first digits are odd and two are even; so there are a total of 3 × 2 2 4 + 2 × 2 8 0 = 1 2 3 2 numbers satisfying the conditions.