Puzzle- the next one

Question: A pipe ‘A’ can fill a tank in 10 hours and another pipe ‘B’ can empty it in 20 hours. How many hours will it take for an empty tank to be filled if both the pipes are opened simultaneously?





Ans: 20 hours.



Clue 1: Consider filling to be +ve and emptying to be –ve.

So Work done is 1/10-1/20=1/ 20.

Work is always inversely proportional to time. So the time taken is 1/20.



Clue 2: If you get the answer to be -1/20, it means the tank will get emptied in 20 hours…