Pipes A and B can fill a tank in 45 minutes and 36 minutes. Both are opened together, and B is closed at some point so that the tank is full in 30 minutes. How many minutes before the tank is full was B closed?
Correct answer C. 18 minutes
Explanation
A works 30 minutes and fills 30/45 = 2/3. B fills 1/3, which takes 12 minutes, so B was closed 30 − 12 = 18 minutes before the tank was full.
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