Answer
$S=9pq$
Work Step by Step
$S$ is directly proportional to the product of $p$ and $q$ so the equation that relates the variables is:
$A=k(pq)$ where k = constant of proportionality.
If p = 4 and q = 5, then S = 180. Substitute these values into the equation above to have:
$S=k(pq)
\\180 = k(4)(5)
\\180=20k
\\\frac{180}{20}=k
\\9=k$
The constant of proportionality is $9$. Thus, the equation that relates S, p, and q is:
$S=9pq$