Maths Olympiad Prep

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Problem 465

Number theory Difficulty 2.7 Find the answer CEMC Fermat

The remainder when 111 is divided by 10 is 1. The remainder when 111 is divided by the positive integer nn is 6. How many possible values of nn are there?

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

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Official solution

Since the remainder when 111 is divided by nn is 6, then 1116=105111-6=105 is a multiple of nn and n>6n>6 (since, by definition, the remainder must be less than the divisor). Since 105=357105=3 \cdot 5 \cdot 7, the positive divisors of 105 are 1,3,5,7,15,21,35,1051,3,5,7,15,21,35,105. Therefore, the possible values of nn are 7,15,21,35,1057,15,21,35,105, of which there are 5.

Source: Omni-MATH, licensed Apache-2.0. Statement reproduced verbatim; metadata (topic, difficulty, ordering) added by this project.