To find radius $ r $ use formula:
$$ V = r ^{ 2 } \cdot h \cdot \pi$$After substituting $ V = 1000 $ and $ h = 20 $ we have:
$$ 1000 = 20 \cdot r ^{ 2 } \cdot \pi $$$$ r ^{ 2 } = \dfrac{ 1000}{ 20 \, \pi } $$$$ r ^{ 2 } = 318.3102 $$$$ r = \sqrt{ 318.3102 } $$$$ r = 17.8412 $$