Answer & Explanation
C) 51 min
The work to be done = Capacity of reservoir = 30 litres.
1st Minute inlet pipe opened 5 lit filled
2nd minute inlet pipe closed; outlet pipe opened 4 lit emptied
In 2 minutes (5 litres - 4 litres = 1lit) is filled into the reservoir.
It takes 2 minutes to fill 1lit it takes 50 minutes to fill 25 litres into the reservoir.
In the 51st minute inlet pipe is opened and the reservoir is filled.