Question 252745
A cyclist bikes x distance at 10 miles per hour and returns over the same path at 8 miles per hour. What is the cyclist's average rate for the round trip in miles per hour?
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d= rt --> t = d/r
t = x/10 + x/8 = 9x/40
r = 2d/t = 2x/t --- 2x for the round trip
r = (2x/1)/(9x/40) = 80x/9x
r = 80/9 = 8.888... mph
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It's not the average, 9 mph, because he spent more time going 8 mph than he did going 10 mph.
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That's the basis of the famous Michelson-Morley experiment that disproved the existence of the "ether" is space.