DIRECTION for the question: Solve the following question and mark the best possible option.

Two pipes A and B can fill a tank in 15 hours and 20 hours respectively while a third pipe C can empty the full tank in 25 hours. All the three pipes are opened in the beginning. After 10 hours, C is closed. In how much time will the tank be full?

Let us assume the capacity of the tank to be 300 litres i.e. LCM of 15, 20 and 25.
This gives the efficiencies of A, B and C to be 20, 15 and 12 litres per hour respectively.
For the first 10 hours, all of the pipes worked together so they filled 10*(20 + 15 - 12) = 230 litres of water.
Water to be filled = 300 - 230 = 70 litres and the total efficiency after 10 hrs is 20+15 = 35 litres/hour. So, it will take 2 more hours to fill the remaining tank.
Total time taken to fill the tank = 10 + 2 = 12 hrs.