Awoman can row a boat at 7.80 km/h in still water. (a) if she is crossing a river where the current is 3.90 km/h, in what direction must her boat be headed if she wants to reach a point directly opposite her starting point? express your answer as an angle with respect to the line perpendicular to the river, positive if down the river. (b) if the river is 7.80 km wide, how long will she take to cross the river? (c) suppose that instead of crossing the river she rows 3.90 km down the river and then back to her starting point. how long will she take? (d) how long will she take to row 3.90 km up the river and then back to her starting point? (e) in what direction should she head the boat if she wants to cross in the shortest possible time, and (f) what is that time?