Question 376885: Flying east between two cities, a planes speed is 380 mph. On the return trip, it flies at 420 mph. Find the average speed over the whole round trip.(the answer is not 400 mph)
Answer by Alan3354(69443)   (Show Source):
You can put this solution on YOUR website! d = rt
t = d/r
t = d/380 + d/420 = 21/7980 + 19/7980 = 2/399 for distance of 2d
r = 399 mph average
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Average round trip rate = 2r1*r2/(r1 + r2)
The average speed is always less than the average of the 2 speeds, because more time is spent moving at the lower speed. If the times were equal, it would be the average of the 2 rates.
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This is the basis of the Michelson-Morley experiment which disproved the existence of "the ether", in the 1940's, I think.
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I looked it up, it was in 1887.
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