Two vertical tanks placed on a platform are connected by a horizontal pipe having a $5$ cm inside diameter. Each tank is $2$ m deep and has a diameter of $1$ m$.$ At first, the valve on the pipe is closed and one tank is full while the other one is empty. When the valve is opened, the average velocity $($m$/$s$)$ through the pipe is given by v $=2*$sqrt$($h$)$ where h is the difference between the levels in the two tanks $($m$).$ Calculate the time for the levels in the two tanks to become equal. Answer: $4\mathrm{.}7$ min