Answer: $36$


Using logic
As the product of each pair has the same value, this value must be product of the smallest and largest numbers, that is $5\times72$.

So the number which is paired with $10$ is $5\times72\div10$, that is $36$.


Using prime factors
Only four have a factor of $5$ so each pair must contain one of these


$5$ has no factors of $2$ or $3$, so pairs with $72$, which has the highest powers of $2$ and $3$ 

To get $2^3$ and $3^2$, $10$ pairs with $36$


The others can also be paired up: