Correct Answer - Option 4 : 12 hrs
Given:
Pipes A and B can fill a tank in 20 hrs and 15 hrs.
Pipe C can empty the tank in 30 hours
Calculation:
Let the total work in be 60 units
⇒ Efficiency of Pipe A filled the tank = (60 units)/20 hrs = 3 units/hr
⇒ Efficiency of Pipe B filled the tank = (60 units)/15 hrs = 4 units/hr
⇒ The efficiency of Pipe C empty the tank = (60 units)/30 hrs = 2 units/hr
If all three pipes are open at the same time,
⇒ (A + B - C) = 3 + 4 - 2 = 5 units/hrs
⇒ (60 units)/(5 units/hrs) = 12 hrs
∴ The time is taken to fill the tank is 12 hrs