SOLUTION: A car travels 120 miles per hour from point A to B at 30mph. Then, travels back at 40 mph. What was the cars average speed for the whole trip?

Question 641718: A car travels 120 miles per hour from point A to B at 30mph. Then, travels back at 40 mph. What was the cars average speed for the whole trip?
Answer by Alan3354(69443) (Show Source): You can put this solution on YOUR website!
A car travels 120 miles per hour from point A to B at 30mph. Then, travels back at 40 mph. What was the cars average speed for the whole trip?
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Travels at 40 mi/hr for how long, or how far?
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A car travels 120 miles per hour from point A to B at 30mph.
What speed is it? 120 or 30?
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I think you meant:
A car travels 120 miles from point A to B at 30mph. Then, travels back at 40 mph. What was the cars average speed for the whole trip?
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t = d/r = 120/30 + 120/40
t = 7 hrs
avg = 240/7 mi/hr
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The distance for a round trip is not relevant.
RT avg speed = 2*30*40/(30+40) = 240/7
It's similar to parallel resistance or parallel flows, but with with a factor of 2.

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