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] $13\tfrac{1}{3}\ hours$