A problem on combinations

Find the number of odd integers between 3000 3000 and 8000 8000 in which no digit is repeated.


The answer is 1232.

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1 solution

Chris Lewis
Apr 8, 2021

The numbers we're interested in begin with one of 3 , 4 , 5 , 6 , 7 3,4,5,6,7 (we trivially discount 8000 8000 ). They end in an odd digit, one of 1 , 3 , 5 , 7 , 9 1,3,5,7,9 .

If the first digit is odd , there are 4 4 choices for the final digit; then 8 8 choices for the second and 7 7 for the third digit, for a total of 4 × 8 × 7 = 224 4\times 8\times 7=224 numbers.

If the first digit is even , there are 5 5 choices for the final digit; then 8 8 choices for the second and 7 7 for the third digit, for a total of 5 × 8 × 7 = 280 5\times 8\times 7=280 numbers.

Three of the possible first digits are odd and two are even; so there are a total of 3 × 224 + 2 × 280 = 1232 3\times 224 + 2\times 280 = \boxed{1232} numbers satisfying the conditions.

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