A pipe canfill a tank in 15 hours. Another pipe can empty the full tank in 20 hours. If both the pipes are opened together the number of hours required to fill the tank is

Solution:

Let total capacity of the tank be 60 units [LCM of 15,20]

First pipe fills 60 units (full tank) in 15 hrs. Hence in 1 hr. it fills (60/15) =4 units

Second pipe empties 60 units (full tank) in 20 hrs. Hence in 1 hr. it empties (60/20)units = 3 units or fills (-)3 units.

If both the pipes are opened simultaneously, total tank filled in 1 hr = 4 + (-)3 units = 1 unit.

So to fill full capacity (60 units), time taken = (60/1) = 60 hrs.