A ship takes 4 hours to sail from port A to port B, and 5 hours to return from port B to port A. It is known that the speed of the ship in still water is constant, so how many hours does it take for a piece of wood to float from port A to port B?
一艘轮船从甲港到乙港需航行4小时，从乙港返回甲港要用5小时．已知船的静水速度不变，那么，一块木板从甲港漂到乙港要用多少小时？
<result>40</result>