There is a boat going from point A to B. It is 180 km from A to B. From point A to B is going downstream, so it only takes 10 hours. To get from point B to A, is upstream so it takes 15 hours. It rains so the river moves faster, and point A to B only takes 9 hours now. Now, how long will it take to get from point B to point A? Can you please help me with this? Explain how to do this problem.