QA – Placement Quizzes | Pipes and Cisterns | Question 2

Two pipes A and B attached to a swimming pool can fill the pool in 20 minutes and 30 minutes respectively working alone. Both were opened together but due to malfunctioning of motor of pipe A, it had to be shut down after two minutes but B continued to work till the swimming pool was filled completely. Find the total time taken to fill the pool.
(A) 20
(B) 22
(C) 25
(D) 27


Answer: (D)

Explanation: Let the capacity of the pool be LCM(20, 30) = 60 units.
=> Efficiency of pipe A = 60 / 20 = 3 units / minute
=> Efficiency of pipe B = 60 / 30 = 2 units / minute
=> Combined efficiency of pipe A and pipe B = 5 units / minute
Now, the pool is filled with the efficiency of 5 units / minute for two minutes.
=> Pool filled in two minutes = 10 units
=> Pool still empty = 60 – 10 = 50 units
This 50 units is filled by B alone.
=> Time required to fill these 50 units = 50 / 2 = 25 minutes
 
Therefore, total time required to fill the pool = 2 + 25 = 27 minutes

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