Question
Pipe A fills a cistern in 6 hours, but pipe B can drain
the full cistern in 9 hours. If both pipes operate together, how long will it take to fill the cistern?Solution
Let the total capacity be 18 litres {LCM (6 and 9)} Efficiency of filling pipe A = (18/6) = 3 units/hour Efficiency of emptying pipe B = (18/9) = 2 units/hour Combined working rate = 3 – 2 = 1 unit/hour Time required to fill the cistern = (18/1) = 18 hours
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