Pipes A and B fill a tank in 12 hours, B and C in 15 hours, and A and C in 20 hours. How long would A, B and C together take?
Correct answer B. 10 hours
Explanation
Adding pairs: 2(A + B + C) = 1/12 + 1/15 + 1/20 = 12/60 = 1/5. So A + B + C = 1/10 and together they take 10 hours.
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