An open cylindrical tank of diameter $D$ contains a liquid of density $ho$ at a depth $H$. The liquid drains through a hole of diameter $d$ in the bottom of the tank. The velocity of the liquid through the hole is $C \sqrt{ } h$, where $h$ is the depth of the liquid at any time $t$. Derive an equation for the time required for $90 %$ of the liquid to drain out of the tank.