Correct Answer: 45 min.
Explanation:
Part filled in 5 min. = 5 x (1/12 + 1/15) = 5 x 9/60 = 3/4
Part emptied in 1 min. (when all the pipes are opened) = 1/6 - (1/12 + 1/15) = (1/6 - 3/20) = 1/60
Now, 1/60 part is emptied in 1 min. 3/4 part will be emptied in (60 x 3/4) = 45 min.
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