A water tank has two drain pipes.?

Question

With only the first pipe open, the drain will empty in 2 hrs. With only the 2nd open, it takes 3hrs to empty. How long will it take to empty if both pipes are open?

Answer 1

1 hour 12 min.

Answer 2

Lets assume there is 300 gallons of water in the tank. When pipe 1 is open it drains at 150 gal/hr. When pipe two is open it  drains at 100 gal/hr. If both pipes were open it would drain at 250 gal/hr so it would take 1 hr and 12 minutes. This assumes the draining is uniform and not affected by the head above the pipe (although I am not sure that would change the answer).

Answer 3

1 / (1/2 + 1/3) = 1.2 hours = 1 hour 12 minutes.