Alana is skateboarding at 19 km/h and throws a tennis ball at 11 km/h to her friend Oliver who is behind her leaning against a wall. What is the speed of the tennis ball if someone was watching the exchange on the sidewalk?

Alana is moving forward at 19 km/h, while she throws the ball backward at 11 km/h relative to herself. For someone standing on the sidewalk, the ball’s speed is the difference: 19 − 11 = 8 km/h. So the ball moves at 8 km/hr, choice A.

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