Can you please help me solving this physics problem *7th grade*?

This is how it goes: The width of a river is 150 metres and its speed is constant. A fisherman wants to go across the river with a boat which he keeps perpendicular to the both sides of the river by paddling with a speed of 2 metres per second. The river move down the boat on a distance of 75 metres. What is the speed of the river?? please help .....its for a friend. I told her i would help her but now i dont know how to solve it can you please help?? I dont want her to think im silly. Thanks in advance.