Three pipes A, B and C can fill a cistern in 15, 20 and 30 min respectively. They were all turned on at the same time. After 5 minutes the first two pipes were turned off. In what time will the cistern be filled?
Explanation:
Let the work to be done = 60 units.
∴ Efficiency of pipe A = 60/15 = 4 units/min ∴ Efficiency of pipe B = 60/20 = 3 units/min ∴ Efficiency of pipe C = 60/30 = 2 units/min
Work done in 5 minutes = (4 + 3 + 2) × 5 = 45 units.
Work left = 60 – 45 = 15 units.
∴ time taken by Pipe C to complete the remaining work = 15/2 = 7.5 minutes.
∴ Cistern is filled in (5 + 7.5) = 12.5 minutes.
Hence, option (b).
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