Question

Pipes A and B can fill a tank in 20 hrs and 15 hrs respectively. Pipe C can empty the tank in 30 hours. If all the three are open at the same time, then what is the time taken to fill the tank? 
1. 10 hrs
2. 20 hrs
3. 15 hrs
4. 12 hrs

Answer 1

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