6 m = 600 cm, 4 m = 4 x 100 cm = 400 cm
Volume of the rainwater = 600 x 400 x 1 cm^{3}
Let the water level in the cylindrical vessel be h
Volume of the water in cylindrical vessel = πr^{2}h = 3.14 X (20)^{2} x h
According to statement, 600 x 400 x 1 cm^{3} = 3.14 x (20)^{2 }x h
⇒ h = 600 x 400/3.14 x 20 x 20 cm = 191.08 cm