Puzzle Details

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?

Answer: 7 hours and 12 minutes.

Firstly we find a time that both 12 hours and 18 hours would fit in to...

In 36 hours:

6 large pipes can drain 3 reservoirs.
6 small pipes can drain 2 reservoirs.

Therefore, together, 6 large and 6 small pipes can drain 5 reservoirs.

36 hours ÷ 5 = 7.2 hours.

7.2 hours = 7 hours and 12 minutes.

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