# Average speed

Students make the trip to Los Angeles at an average speed of 40mph and the trip back at an average speed of 60 mph. Find their average speed for the entire trip. (It's Not 50 mph)

 colbtech Posts: 733, Reputation: 331 Senior Member #2 Aug 23, 2006, 12:31 AM
I think that I am missing something here!
If the answer is not 50, then there must be other factors to give a different answer.

From your question, the distance and time is irrelevant. It is a simple matter of adding the two average speeds and dividing by 2, which gives an answer of 50. ?
 Dr D Posts: 702, Reputation: 630 Senior Member #3 Aug 23, 2006, 10:34 AM
Just enter any random distance for the trip; say 200 miles. One way at 40 mph avg = 5 hours. On the return trip at 60 mph avg = 3.333 hours. Total round trip of 400 miles took 8.333 hours. 400/8.333 = 48mph avg.
 kp2171 Posts: 5,390, Reputation: 8183 Uber Member #4 Aug 23, 2006, 11:17 AM
Dr d is correct.

The assumption that is wrong with adding 40 + 60 / 2 is that you are driving a fixed distance (point A to LA and then back), not a fixed time (one hour there and the same time back). Does it make sense that you are going to be driving longer at 40 mph than on the way home?

So since the average speed is the overall distance divided by time, you need to figure out how long you would be in the car both ways. Dr d's example works.

Just to give another ex, if your distance was 500 miles to LA, you'd drive 40 mph, which gives 500miles * (1/40 mph) = 12.5 hours to go 500 miles.

Then if you drive the same 500 miles back at 60mph you'd get 500 miles * (1/60mph) = 8.33 hours to go 500 miles.

So 1000 miles / (12.5+8.3 hours) = 1000/20.8 = 48.1 miles per hour.

Im guessing this is for a physics class you are taking. The hardest part of this problem is realizing you needed to simply start with a theoretical distance and use the speed to come up with time.
 BigD Posts: 9, Reputation: 1 New Member #5 Aug 23, 2006, 12:49 PM
Thank You every body for helping. I just realizes how easy it is if you think it through more carefully. Thanks again
 colbtech Posts: 733, Reputation: 331 Senior Member #6 Aug 24, 2006, 12:31 AM
Quote:
 Originally Posted by BigD Thank You every body for helping. I just realizes how easy it is if you think it through more carefully. Thanks again

Yes, thanks folks! Makes sense your way.
 lilybean Posts: 1, Reputation: 1 New Member #7 Jul 4, 2009, 01:19 PM
I drive to L.A. At 40 mph and back from L.A. At 60 mph. What's my average speed?
 Unknown008 Posts: 8,147, Reputation: 3745 Uber Member #8 Jul 4, 2009, 09:01 PM
Lilybean, you should've posted in another thread, one that you have created by clicking on 'Ask about Math and Sciences'

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