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A large fresh water reservoir has two types of drainage system. Small pipes and large pipes. 6 large pipes, on their own, can drain the reservoir in 12 hours. 3 large pipes and 9 small pipes, at the same time, can drain the reservoir in 8 hours. How long will 5 small pipes, on their own, take to drain the reservoir? [Ref: ZHRT] © Kevin Stone21 hours and 36 minutes. The exact size of the pipes and reservoir does not matter. If we label the large pipes L, the small pipes S and if the reservoir has a total of T litres, then: T ÷ 6L = 12 which means L = T ÷ 72 and T ÷ (3L + 9S) = 8 which means that S = T ÷ 108 we want T ÷ 5S which gives 108 ÷ 5 hours = 21.6 hours = 21 hours and 36 minutes.
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