Two pipes can fill a cistern separately in 6 minutes and 9 minutes, respectively. If both pipes are opened at the same time, how long will it take to fill the cistern?

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• A 1.2 minutes
• B 2.5 minutes
• C 3.6 minutes
• D 4.5 minutes

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Answer: C. 3.6 minutes

In 1 minute the pipes fill 1/6 + 1/9 = 5/18 of the cistern. Time = 18/5 = 3.6 minutes.

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