alleryTwo pipes A and B fill a tank in 4 and 6 hours. A drain C empties it in 12 hours. With all three open, how long to fill?
A$6$ hours
B$5$ hours
C$4$ hours
D$3$ hours
Answer & Solution
Correct answer: D. $3$ hours
**Rates:** A = 1/4, B = 1/6, C = -1/12 (emptying).
Combined = 3/12 + 2/12 − 1/12 = 4/12 = 1/3 per hour. Time = 3 hours.
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