Answer
$ r(h) = \frac{5\sqrt{\pi h}}{\pi h} $
Work Step by Step
Let $r$ be the radius of the cylinder and $h$ be the height of the cylinder.
Volume: $V=\pi r^2 h$
Substituting $V=25$ and isolating $r$:
$$ \begin{align*}
\displaystyle V& =\pi r^2 h \\\\
25& =\pi r^2 h\\\\
r^2& =\frac{25}{\pi h} \\\\
r& = \sqrt{\frac{25}{\pi h}}\\\\
r& = \frac{5\sqrt{\pi h}}{\pi h}\\\\
\end{align*} $$
