Two ingoing pipes can fill a tank in 4 and 6 hours, respectively. An outgoing pipe is attached to these two pipes and thus the tank was filled in 3 hours. In 60 hours the outgoing pipe alone can empty how many tanks?

Question: Two ingoing pipes can fill a tank in 4 and 6 hours. An outgoing pipe is attached to these two pipes and thus the tank was filled in 3 hours. In 60 hours the outgoing pipe alone can empty how many tanks?

Solution:
Let the outgoing pipe take P hours to empty the tank.

so, in one hour total fillup = 1/4 + 1/6 - 1/P
= (5P - 12)/12P

∴ 12P/(5P - 12) = 3
or, 12P = 15P - 36
or, 3P = 36
∴ P = 12

in 60 hours it can empty = 60/12 = 5 tanks