To find radius $ r $ use formula:
$$ V = r ^{ 2 } \cdot h \cdot \pi$$After substituting $ V = 320 $ and $ h = 5 $ we have:
$$ 320 = 5 \cdot r ^{ 2 } \cdot \pi $$$$ r ^{ 2 } = \dfrac{ 320}{ 5 \, \pi } $$$$ r ^{ 2 } = 101.8592 $$$$ r = \sqrt{ 101.8592 } $$$$ r = 10.0925 $$