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Charlie rides his bicycle down a hill at 30 kilometers per hour. Then he turns around and rides back up the hill at 6 kilometers per hour to his starting point. What is Charlie's average speed for the whole trip down and back up the hill?

 Jun 26, 2020
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The distance is the same   d = rate * time     it is the TIME that is different

 

d = 30 k/h * t1

d = 6k/h *   t2        and they are equal 

 

d/30  = t1    and   d/6 =t2       

Total distance is 2 d

 

2d     =  Average *(t1 +t2)

2d = average * (d/30 + d/6)

2d = average ( (d+5d)/30)

2d = average * (6d/30)

2d * 30/6d = average

2*30/6 = average  = 10 k/hr

 Jun 26, 2020

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