SOLUTION: You are traveling to grandmother's house and get stuck in traffic so that you averaged only 25 mph on your way there. For you entire trip (there and back and taking the same route)

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Question 711236: You are traveling to grandmother's house and get stuck in traffic so that you averaged only 25 mph on your way there. For you entire trip (there and back and taking the same route) you averaged 45 mph. What was your average speed on your trip home from grandma's? (the answer is not 65 mph)
Answer by Alan3354(69443) (Show Source):
You can put this solution on YOUR website!
You are traveling to grandmother's house and get stuck in traffic so that you averaged only 25 mph on your way there. For you entire trip (there and back and taking the same route) you averaged 45 mph. What was your average speed on your trip home from grandma's? (the answer is not 65 mph)
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65 is one of many speeds that are not the answer, I don't see why you mentioned it.
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avg = 2*25*45/(25+45) = 2250/70
avg = 32 1/7 mi/hr
=~ 32.143 mi/hr
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Maybe you meant to say not 35 mi/hr
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The reason it's less than the average of the 2 speeds is more time is spent at the lower speed.
The formula is similar to parallel resistors and parallel flows, but with an additional factor of 2.