It takes 12 hours for a ship to sail from A in the upper reaches of a river to B in the lower reaches, and it takes less than 12 hours to return from B at a constant speed. The speed of this river is 3 km/h, and the still water speed v of the ship remains unchanged. What conditions does v meet? Solution: Let the stroke be S =10 (3+V) = 30+10Vs.

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