The speed of the boat in still water is 4 times that of the current. It takes 6 hours to go from point A upstream to point B downstream. How many hours does it take to return from point B to point A?
船在静水中速度是水流的4倍，它从上游甲地到下游乙地共用6小时，从乙地返回甲地需要几小时？
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