Answer
$q=\displaystyle \frac{pf}{p-f}$
Work Step by Step
Get rid of fractions (multiply with LCD=pqf)
$ qf+pf=pq\qquad$ ... add $-qf$
$ pf=pq-qf\qquad$ ... factor out q
$ pf=q(p-f)\qquad$ ... divide with $(p-f)$ to isolate $q$
$\displaystyle \frac{pf}{p-f}=q$
$q=\displaystyle \frac{pf}{p-f}$