SOLUTION: Flying east between two cities, a plane's speed is 380 mph. On the return trip it flies at 420 mph. Find the average speed over the whole round trip. (NO, the answer is not 400

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Question 252822: Flying east between two cities, a plane's speed is 380 mph. On the return trip it flies at 420 mph. Find the average speed over the whole round trip. (NO, the answer is not 400 mph.)
Found 2 solutions by Alan3354, richwmiller:
Answer by Alan3354(69443) (Show Source):
You can put this solution on YOUR website!
Here's a similar problem I did yesterday.
If you need further help, or to check your work, email via the thank you note.
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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.

Answer by richwmiller(17219) (Show Source):
You can put this solution on YOUR website!
No, it isn't 400 but the difference is not worth the bother. I doubt the speedometers are that accurate anyways, especially if they are analog.
r=distance/time
time=d/380+d/420
distance=2d
r=2d/(d/380+d/420)
r=399