Maths Olympiad Prep

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Number theory Difficulty 4.7 AIME Find the answer Italy

Problem:

In how many ways can the digits 11, 22, 44, 77 and 99 be ordered so as to form a five-digit number divisible by 1111?

Pick one

Solution

Solution:

The answer is (D). A number is divisible by 1111 if and only if the difference between the sum of the digits in odd position and the sum of the digits in even position is divisible by 1111.

Let aa be the sum of the 33 digits in odd position and bb the sum of the 22 digits in even position. Since a+b=1+2+4+7+9=23a+b=1+2+4+7+9=23 is odd, aba-b will also be odd and hence different from 00. Moreover, from the fact that bb is at most 7+9=167+9=16 and at least 1+2=31+2=3, it follows that ab=(23b)b=232ba-b=(23-b)-b=23-2b lies between 9-9 and 1717. If aba-b must be divisible by 1111, the only possibility is therefore that it equals 1111, but then 11=ab=232b11=a-b=23-2b, from which b=6b=6. The only pair of digits among those considered giving sum 66 is {2,4}\{2,4\}.

In conclusion, one obtains a multiple of 1111 every time, and only when, the digits in even position are 22 and 44. In total there are 1212 combinations, because there are 22 ways to fix the order of the digits in even position and 3!=63!=6 ways to fix the order of the other 33.

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Source: MathNet, licensed CC-BY-4.0. Statement translated into English from it; metadata (topic, difficulty) added by this project.