the fill pipe for a tank can fill the tank in 4 hours and the drain pipe can drain in 2 hours. If both pipes are accidentally opened, how long will it take to empty a half-filled tank?

Answer 1

Let V = tank volume
Fill rate = 1/4 V per hr
Drain rate = 1/2 V per hr
Net loss rate = 1/2 - 1/4 = 1/4 V/hr
Drain time = (V/2)/(1/4 V/hr) = 2 hr