Answer
The answer is $p=\frac{fq}{q-f}$.
Work Step by Step
The given expression is
$\frac{1}{p}+\frac{1}{q}=\frac{1}{f}$
Subtract $\frac{1}{q}$ from both sides.
$\frac{1}{p}+\frac{1}{q}-\frac{1}{q}=\frac{1}{f}-\frac{1}{q}$
Simplify.
$\frac{1}{p}=\frac{1}{f}-\frac{1}{q}$
Subtract on the right hand side.
$\frac{1}{p}=\frac{q-f}{fq}$
Reciprocate both sides.
$\frac{p}{1}=\frac{fq}{q-f}$
$p=\frac{fq}{q-f}$.
