It takes 1 hour for a small boat to travel downstream from port A to port B. After doubling the speed of the water, it takes 50 minutes to sail from port A to port B. After the speed of the water increases, how much does it take to return from port B to port A?
一只小船从甲港到乙港顺流航行需1小时，水流速度增加一倍后，再从甲港到乙港航行需50分钟，水流速度增加后，从乙港返回甲港需航行多少．
<result>2.5</result>