Question 547710
A person travels 136 miles from one town to a city at an average speed of 77 mph and then returns home at an average speed of 56 mph. 
What was the average speed over the total distance covered?
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Avg speed of a round trip is 2*R1*R2/(R1 + R2) R1 & R2 are the 2 rates.  
I worked this out some time ago while looking at the Michelson-Morley experiment.
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It's similar to parallel resistors and parallel flows, but with a factor of 2.
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Avg speed of the round trip = 2*77*56/(77+56)
=~ 64.84 mi/hr