Pipe A fills a tank in 6 hours and pipe B empties it in 8 hours. Both are opened on an empty tank, and after 12 hours B is closed. What is the total time taken to fill the tank?
Correct answer B. 15 hours
Explanation
Net rate with both open = 1/6 − 1/8 = 1/24, so 12 hours fill 1/2. A alone fills the other half in 3 hours. Total = 15 hours.
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