Two pipes can fill a tank in 9 hours and 12 hours. Both are opened together, and after 3 hours the first pipe is closed. How much more time will the second pipe take to fill the tank?
Correct answer C. 5 hours
Explanation
In 3 hours both fill 3 × 7/36 = 7/12. The remaining 5/12 at 1/12 per hour takes 5 hours.
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