A cistern has two pipes. One can fill it with water in 8 hours and other can empty it in 5 hours. In how many hours will the cistern be emptied if both the pipes are open together when \frac{3}{4} of the cistern is already full of water?
[A] 3\tfrac{1}{3}\ hours
[B] 6\ hours
[C] 10\ hours
[D] 13\tfrac{1}{3}\ hours
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