A boat that can travel at 4.0 km/h in still water crosses a river with a current of 1.8 km/h. At what angle must the boat be pointed upstream to travel straight across the river? In other words, in what direction is the velocity of the boat relative to the water? I have already found the answer due to the equation: sin-1(1.8/4) The answer is 27 degrees upstream. Reading through my book, I understand this has something to do with the boat's velocity equaling the vector sum of it's velocity in relation to the water??? Something like that. Can any of you clarify the relationship between the boat and water speeds, and why this equation yielded a correct answer? Thanks!