Question on the 2020 RMO.

contest-math

In the “expression”
each of the twelve @ symbols is replaced with either $\times$ or $÷$ such that the value of the resulting
expression is an integer.

1@2@3@4@5@6@7@8@9@10@11@12@13

Find the greatest common factor of all such integer values.

Best Answer

First every such integer is divisible by $7, 11, 13$.

Highest power of $2$ of $13!$ is even, so you need to find an integer product such that it's odd. This can be easily done.

Then the highest power of $3$ of $13!$ is odd, so you need to find an integer product such that it's divisible by $3$ but not by $9$. This is easy too.

The highest power of $5$ of $13!$ is $2$. It's easy to find one integer product that's not divisible by $5$.

Therefore the $\gcd$ of all integer products is $3\times 7 \times 11 \times 13 = 3003$.