Answer
$\frac{1\text{ professors}}{5\text{ graduate assistants}}$
Work Step by Step
4 professors to 20 graduate assistants --> $\frac{4\text{ professors}}{20\text{ graduate assistants}} =\frac{4\text{ professors}\div4}{20\text{ graduate assistants}\div4} = \frac{1\text{ professors}}{5\text{ graduate assistants}}$