Question 489531
The fill pipe for a tank can fill the tank in 4 hours, and the drain pipe can drain it in 2 hours. 
If both pipes are accidentally opened, how long will it take to empty a half-filled tank?
:
Let t = time to empty the tank with both valves open
Let an empty tank = 1
then a half empty tank = .5
:
{{{t/2}}} - {{{t/4}}} = .5
multiply 4, results:
2t - t = 4(.5)
t = 2 hrs to empty the tank