A large fresh water reservoir has two types of drainage system. Small pipes and large pipes.
[Ref: ZNEC] © Kevin Stone
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?
Hint: It doesn't matter how large the pipes are .
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