Pipes A and B can fill a tank in 20 minutes and 30 minutes respectively. Both are opened together, and after 6 minutes pipe A is closed. How much more time will B take to fill the tank?
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Answer: Option B — 15 min
Detailed Solution
In 6 minutes both fill .
The remaining is filled by B in minutes.
✓ Check
B works 21 min ( ) and A works 6 min ( ); .⚠ Common trap
(D) 21 min — gives B's total running time instead of the extra time.Quick tip
Find the part done together, then divide the rest by the remaining pipe's rate
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Ask a doubtMore Pipes & Cisterns Questions
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Pipes & Cisterns — Preparation
This question tests your understanding of Pipes & Cisterns, a key topic in the Quantitative Aptitude section. The detailed solution above uses a step-by-step approach to show the reasoning, and where available, a shortcut that helps you solve similar questions in under 90 seconds during the actual exam.
Pipes and cisterns questions are an application of time and work. An inlet pipe fills a tank and does positive work, while an outlet pipe or leak empties it and does negative work. If an inlet pipe fills a tank in a hours, it fills 1/a of the tank each hour. If an outlet pipe empties it in b hours, it removes 1/b each hour. Questions involve several pipes opened together, pipes opened or closed partway through, leaks that slow down filling, and pipes with different filling rates. The LCM method works especially well. Take the tank's capacity as the LCM of the given times, find each pipe's rate in units per hour (negative for outlet pipes and leaks), and add the rates. This avoids fractions entirely. Always check whether the question asks for the time to fill or the time to empty. Practise the pipes and cisterns questions below, each with a step-by-step solution using the LCM method.