A hemi-spherical tank of radius 2 m is initially full of water and has an outlet of `12c m^2` cross-sectional area at the bottom. The outlet is opened at some instant. The flow through the outlet is according to the law `v(t)=sqrt(0. 62gh(t)),` where `v(t)` and `h(t)` are, respectively, the velocity of the flow through the outlet and the height of water level above the outlet and the height of water level above the outlet at time `t ,` and `g` is the acceleration due to gravity. Find the time it takes to empty the tank.