Simon is riding a bike at 12 kilometers per hour away from his friend Keesha. He throws a ball at 5 kilometers per hour back to Keesha, who is standing still on a sidewalk. How fast would Keesha say the ball is traveling?

Take the direction away from Keesha as positive. Simon moves at 12 km/h in that direction, while the ball is thrown at 5 km/h back toward Keesha relative to Simon. So the ball’s speed relative to the sidewalk is 12 − 5 = 7 km/h away from Keesha. Keesha would say the ball is traveling at 7 kilometers per hour, choice D.

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