Thuy is on a west bank of a river that flows from north to south at speed of 5m/s She is easily able to swim at a constant speed of 1m/s. The river is 30m wide.
a) On what bearing should Thuy swim to end up on the east bank at a point directly opposite her starting position?
b) How long will it take Thuy to reach the other side of the river?

You must set a coordinate system (Y - X axes ) where Thuy is at zero point. Analyse velocities on y - x components so that Vy(of Thuy on y) = 0 m/s . Remember that for X component: Vx(of Thuy on x) = (1 m/s)*cos(θ) and for Y component: Vy(of Thuy on y) = (1m/s)*sin(θ) + V(of river) = 0 . Solve for θ and then use t=X/V for time.