How do I do this problem using a transient mass balance?

1. (10 pts) The bottom of a round tank of diameter D filled with water to an initial height H0 is connected a horizontal pipe of diameter d and length L. The top of the tank and the pipe outlet are both open to the atmosphere. At time t=0 a valve is opened. You may assume that minor losses and the kinetic energy term in the mechanical energy balance are both negligible. Find (symbolically) the time it takes for half of the tank to drain (t1/2) if a) Flow in the pipe is assumed to be laminar b) Flow in the pipe is assumed to be turbulent, with Re so high that friction factor f is basically constant 2. (5 pts) One of the oldest methods of measuring time is with a water clock. The simplest form of a water clock