An ingoing pipe can fill a tank in 5 hours while an outgoing pipe can pour all the water in 15 hours. If both the pipes are open at once, how much time will it take to fill the whole tank?

Question: An ingoing pipe can fill a tank in 5 hours while an outgoing pipe can pour all the water in 15 hours. If both the pipes are open at once, how much time will it take to fill the whole tank?

Solution:
in one hour,
ingoing fillup = 1/5 of the tank
outgoing pour = 1/15 of the tank water
∴ total fillup in one hour = 1/5 - 1/15
= 2/15

total time to fill the tank is = 15/2 = 7.5 hours.