Two pipes A and B can fill a cistern in 12 minutes and 15 minutes respectively. There is also a waste pipe C. If all three pipes are opened together, the cistern is full in 20 minutes. How long will C take to empty the full cistern?
Explanation:
Total capacity = LCM(12, 15, 20) = 60 units. Efficiency of A = 5, B = 4. Combined efficiency of A+B+C = 3. Efficiency of C = 3 - (5 + 4) = -6. Thus, C empties 6 units/minute. Time taken by C to empty the full cistern = 60 / 6 = 10 minutes.
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