To find height $ h $ use formula:
$$ V = \dfrac{ r ^{ 2 } \cdot h \cdot \pi}{ 3 } $$After substituting $ V = 50 $ and $ r = 20 $ we have:
$$ 50 = \dfrac{ 20 ^{ 2 } \cdot h \cdot \pi}{ 3 } $$$$ 50 \cdot 3 = 20 ^{ 2 } \cdot h \cdot \pi $$$$ 150 = 400 \cdot h \cdot \pi $$$$ h = \dfrac{ 150 }{ 400 \, \pi} $$$$ h = 0.1194 $$