A man wishes to cross a river of width 120 m by a motorboat. His rowing speed in still water is 3 m/s and his maximum walking speed is 1 m/s. The river flows with velocity of 4 m/s.
a. - Find the path which he should take to get to the point directly opposite to his starting point in the shortest time.
b. - Also, find the time which he takes to reach his destination.