Interesting Sum

Find the sum of all positive two-digit integers that are divisible by each of their digits.


The answer is 630.

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1 solution

Maggie Miller
Jul 19, 2015

Note 0 0 doesn't divide any number except itself (depending on definition), so a number that is divisible by all of its digits cannot have as a digit.

We recall some rules of divisibility for digits (for two-digit numbers):

  1. All integers are divisible by 1 1 .
  2. The second digit is even.
  3. The sum of the digits is divisible by 3.
  4. If the first digit is even, the second is 4 4 , 8 8 , or 0 0 . If the first digit is odd, then the second is 2 2 or 6 6 .
  5. The second digit is 0 0 or 5 5 .
  6. The digits sum to a multiple of 3 3 and the second digit is even.
  7. The only two-digit multiples of 7 7 with 7 7 as a digit are 70 70 and 77 77 .
  8. The only two-digit multiples of 8 8 with 8 8 as a digit are 48 48 , 80 80 and 88 88 .
  9. The only two-digit multiples of 9 9 with 9 9 as a digit are 90 90 and 99 99 .

Combining all these rules give 14 two-digit numbers: 11 , 12 , 15 , 22 , 24 , 33 , 36 , 44 , 48 , 55 , 66 , 77 , 88 , 99. 11,12,15,22,24,33,36,44,48,55,66,77,88,99.

The sum of these numbers is 630 \boxed{630} .

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