Question

Two skaters, Alan and Betsy, move toward each other on a very smooth skating rink. Their masses and speeds are respectively: 60 kg; 4.0 m/s and 50 kg; 0.4 m/s. After the skaters collide and use their hands to push each other away, Alan is moving backwards at 2.0 m/s. Taking the direction of Alan's initial velocity as positive, find Betsy's velocity vf after the collision.