Question
Two pipes A and B can fill a tank in 15 hours and 20
hours, respectively. If both pipes are opened together and then pipe A is closed after 5 hours, how long will it take for pipe B to fill the tank?Solution
Work done by pipe A in one hour = 1/15. Work done by pipe B in one hour = 1/20. In 5 hours, work done by A = 5 * (1/15) = 1/3. Remaining work = 1 - 1/3 = 2/3. Now, only pipe B is working. Work done by B in one hour = 1/20. Time taken by B to fill the remaining tank = (2/3) / (1/20) – 5 hours = (2/3) * 20 -5      = 40/3 -5 hours ≈ 8.33 hours. Correct option: d) 8.3 hours
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