For all positive integer values of $p$ and $q$, $2p^2 q$ and $3pq^2$ have a common factor of $pq$.

They will also have an additional common factor of $2$ if $q =2$ and an additional common factor of $3$ if $p=3$.

As the values of $p$ and $q$ are to be chosen from $2, 3$ and $5$, the largest possible value of the highest common factor will occur when $p=3$ and $q=5$.

For these values of $p$ and $q$, $2p^2 q$ and $3pq^2$ have values $90$ and $225$ respectively, giving a highest common factor of $45$.

*This problem is taken from the UKMT Mathematical Challenges.**View the archive of all weekly problems grouped by curriculum topic*

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