Question 484184
Michael  drove to a friend's house at a rate of 40 mph.  He returned by the same route at a rate of 45 mph.  The driving time for the round trip was 4 hours.  What is the distance Michael traveled.
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d/40 + d/45 = 4
45d + 40d = 7200
d = 7200/85 miles
d = 1440/17 miles
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The avg speed for the round-trip is 2*40*45/(40+45)
= 42.353 mph
It's less than the average of 40 & 45 because more time is spent going slower.