There are $\displaystyle x$ number of positive divisors of $\displaystyle 72$, $\displaystyle y$ number of positive divisors of $\displaystyle 900$.

The positive divisors of $\displaystyle 900$ that are not divisors of $\displaystyle 72$ are $\displaystyle z$.

Hoping there's a formula and you're not actually supposed to go over all possible numbers...

Any help appreciated.