Simon is riding a bike at 12 km/h away from his friend keesha. he throws a ball at 5 km/h back to keesha, who is standing still on a sidewalk. how fast would keesha say the ball is traveling?

So there are different ways 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. 

Therefore the answer is: 7 km/hr