Question
A tank can be filled by two pipes, A and B, in 15
minutes and 20 minutes, respectively. Both pipes are opened together, but after 5 minutes, pipe A is closed. How much total time will it take to fill the tank?Solution
Work done by A and B in 1 minute = 1/15 + 1/20 = (4 + 3) / 60 = 7/60. Work done by A and B in 5 minutes = 5 × 7/60 = 35/60 = 7/12. Remaining work = 1 - 7/12 = 5/12. After A is closed, only B is working. Time taken by B to complete the remaining work = (5/12) ÷ (1/20) = (5/12) × 20 = 100/12 = 8.33 minutes. Total time = 5 minutes + 8.33 minutes ≈ 13.33 minutes .
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