To find radius $ r $ use formula:
$$ V = h ^{ 2 } \cdot r \cdot \pi $$After substituting $V = 320\, \text{cm}$ and $h = 5\, \text{cm}$ we have:
$$ 320\, \text{cm} = 25\, \text{cm}^2 \cdot r \cdot \pi $$$$ r = \dfrac{ 320\, \text{cm} }{ 25\, \text{cm}^2 \, \pi} $$$$ r = 4.0744\, \text{cm}^-1 $$