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A motorist is driving at 20m/s{\rm m/s} when she sees that a traffic light 200m{\rm m} ahead has just turned red. She knows that this light stays red for 15s{\rm s} , and she wants to reach the light just as it turns green again. It takes her 1.0s{\rm s} to step on the brakes and begin slowing at a constant deceleration.What is her speed as she reaches the light at the instant it turns green?

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