there are different ways 0f this could be solved. I'll do try to explain it the way I was taught... Simon is riding his bike at 12 km/hr relative to the sidewalk, away from where Keesha is. Simon throws the ball at Keesha, at 5 km/hr. Keesha sees the ball approaching her at (12-5) = 7 km/hr relative to the ground to her. the answer is: 7 km/hr