71. As shown in the figure, river Y is a tributary of the main river X, with the direction of water flow indicated by the arrows. The flow speed of the main river and the tributary is the same, and the still water speed of the boat in the main river and the tributary is also the same. Given $A C=C D$, it takes 6 hours for the boat to travel from $A$ to $B$ via $C$, 8 hours from $B$ to $D$ via $C$, and 5 hours from $D$ to $B$ via $C$. How many hours does it take for the boat to travel from $B$ to $A$ via $C$, and then from $A$ to $D$ via $C$?
Answer: $12 \frac{1}{3}$