• Two friends meet at the Davis water tower. They start biking away from each other at the exact same time: friend A bikes west at a constant speed of 9 miles per hour, and friend B bikes south at a constant speed of 12 miles per hour.
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• (1) Draw a picture showing the starting point and the positions of the two bikers after t hours. 
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• (2) At what rate is the distance between the two bikers changing when they've been biking for precisely one hour? 
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• (3) At what rate is the distance between the two bikers changing when they've been biking for precisely 20 minutes.