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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. 6 small pipes, on their own, can drain the reservoir in 18 hours. How long will 6 large pipes and 6 small pipes take to drain the reservoir?  [Ref: ZNEC] © Mr Stone Hint: It doesn't matter how large the pipes are . 7 hours and 12 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 ÷ 6S = 18 which means S = T ÷ 108 we want T ÷ (6L + 6S)    = T ÷ ((6 x T ÷ 72) + (6 * T ÷ 108))    = T ÷ ((648 x T + 432 x T) ÷ 7776)    = T ÷ ((1080 x T) ÷ 7776)    = 7776 x T ÷ (1080 x T)    = 7776 ÷ 1080    = 7.2 hours    = 7 hours and 12 minutes. QED.