SOLUTION: A car travels 120 miles from A to B at 30 miles per hour but returns the same distance at 40 miles per hour. What is the average speed for the round trip?

Question 260225: A car travels 120 miles from A to B at 30 miles per hour but returns the same distance at 40 miles per hour. What is the average speed for the round trip?
Answer by Alan3354(69443) (Show Source): You can put this solution on YOUR website!
A car travels 120 miles from A to B at 30 miles per hour but returns the same distance at 40 miles per hour. What is the average speed for the round trip?
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120 miles at 30 mph takes 4 hours
120 miles at 40 mph takes 3 hours
A total of 240 miles in 7 hours
--> 240/7 =~ 34.2857
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If you drive an hour at 40 and an hour at 30, the average speed is 35.
It's never the average of the 2 speeds when the distance is the same. It's always less, because more time is spent at 30 mph than at 40 mph.
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This is the basis of the Michelson-Morley experiment that disproved the existence of "the ether."

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