When x=0, T= the time to walk completely by land, because x=0 equals no river, and then I thought about the speed of land. When x=20, T= the time to walk completely by water, because x=20 equals no land, and then I get the speed of water, and then I won't. Personally, I think the topic doesn't give the width of the river. Without a condition, there is no idea behind it. I know the answer.