Permutation of distinct digits

1234567 is not divisible by 11. But we can rearrange its digits to 1537426 so that it is divisible by 11.

Can we also rearrange the digits of the number 12345678 so that the resultant number is again divisible by 11?

Yes No

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1 solution

The rule for divisibility by 11 11 is that the alternating sum of an integer be divisible by 11 11 . As an example, the alternating sum of 8239 8239 is 8 2 + 3 9 = 0 8 - 2 + 3 - 9 = 0 , which is divisible by 11 11 and thus so is 8239 8239 , (indeed 8239 = 11 × 749 8239 = 11 \times 749 ). Now the sum of the digits 1 1 through 8 8 is 36 36 , so one way we can create a number that is divisible by 11 11 is to choose 4 digits that sum to 18 18 , leaving the remaining 4 digits to sum to 18 18 as well, and then alternating these two sets of digits. So choosing 1 , 4 , 5 , 8 1,4,5,8 as one 4-digit set, we know by the alternating-sum rule that 12435687 12435687 will be divisible by 11 11 , since 1 2 + 4 3 + 5 6 + 8 7 = 0 1 - 2 + 4 - 3 + 5 - 6 + 8 - 7 = 0 . The answer is therefore Yes \boxed{\text{Yes}} .

Note: 12435687 = 11 × 1130517 12435687 = 11 \times 1130517 . A potential follow-up question would be to find the probability that a random permutation of 12345678 12345678 is divisible by 11 11 .

A curiously neat 4 35 \dfrac{4}{35} th of the time.

Michael Mendrin - 2 years, 11 months ago

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Oh good, that's the value I got too. I guess I should post the follow-up sometime then. :)

Brian Charlesworth - 2 years, 11 months ago

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