Maths Olympiad Prep

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Number theory Difficulty 5.7 AIME, harder Prove it

## Task A-2.4.

Prove that the sum of all three-digit numbers whose decimal representation consists of three different digits, all different from zero, has at least three different prime divisors.

Solution

## Solution.

Let abc\overline{a b c} be one such number.

Then the numbers acb,bac,bca,cab,cba\overline{a c b}, \overline{b a c}, \overline{b c a}, \overline{c a b}, \overline{c b a} are among the numbers we are adding.

Their sum is

abc+acb+bac+bca+cab+cba=(100a+10b+c)++(100c+10b+a)=222a+222b+222c=222(a+b+c). \begin{aligned} \overline{a b c}+\overline{a c b}+\overline{b a c}+\overline{b c a}+\overline{c a b}+\overline{c b a} & =(100 a+10 b+c)+\ldots+(100 c+10 b+a) \\ & =222 a+222 b+222 c=222(a+b+c) . \end{aligned}

Therefore, the sum of all such numbers is divisible by 222, 1 point

which means it is divisible by 2, 3, and 37.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.