40. At 8:00 AM, the Knight and the Warrior set off from ports $A$ and $B$ respectively, heading towards each other. The Knight arrives at the downstream port $B$, and the Warrior arrives at the upstream port $A$, and both immediately turn around and return. At 10:00 AM, both ships return to their respective starting points for the first time. It is known that the time both ships travel in the same direction is 10 minutes, and the speed of the current is 0.5 meters/second. What is the speed of the Knight in still water? $\qquad$ meters/second.
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6
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