A large fresh water reservoir has two types of drainage system: small pipes and large pipes.
[Ref: ZHRT] © Kevin Stone
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?
21 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
T ÷ (3L + 9S) = 8 which means that S = T ÷ 108
T ÷ 5S which gives 108 ÷ 5 hours = 21.6 hours = 21 hours and 36 minutes.
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