Pipes A and B can fill a tank in 40 minutes and 60 minutes. Both are opened for 12 minutes and then A is closed. How many more minutes will B take to fill the tank?
Correct answer C. 30 minutes
Explanation
Together they fill 1/40 + 1/60 = 1/24 per minute, so 12 minutes fill 1/2. B fills the remaining half in 60/2 = 30 minutes.
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