Question 1008835: Tim drove halfway Indianapolis at 60 mi/h and the rest of the way at 80 mi/h. What was his average speed for the whole trip? Found 2 solutions by ikleyn, Alan3354:Answer by ikleyn(52775) (Show Source):
Let L be the distance Tim covered.
Since Tim drove half of the distance, , at the speed of 60 mph, he spent for it the amount of time / = hours.
Since Tim drove the other half of the distance. , at the speed of 80 mph, he spent for it the amount of time / = hours.
The average speed is the whole distance L divided by the time spent, which is + , or
= = = 68.571 .
The value is called the harmonic mean of values u and v.
Thus the average speed in this problem is the harmonic mean of the given speed 60 mph and 80 mph.
See also the lesson Calculating an average speed: a train going from A to B and back in this site.
You can put this solution on YOUR website! Tim drove halfway Indianapolis at 60 mi/h and the rest of the way at 80 mi/h. What was his average speed for the whole trip?
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For a round-trip, or trip with 2 parts of equal distance:
Avg = 2*60*80/(60+80) = 9600/140
= 480/7 mi/hr
=~ 68.57 mi/hr
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It's not 70 because more time is spent at 60 mi/hr.
