SOLUTION: 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?

Algebra.Com
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(52787)   (Show Source): You can put this solution on YOUR website!
.
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.


Answer by Alan3354(69443)   (Show Source): 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?
----------
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
=====================
It's not 70 because more time is spent at 60 mi/hr.
RELATED QUESTIONS

Elvin drove halfway from ashton to dover at 40 mi/h abd the rest of the way at 60 mi/h.... (answered by Earlsdon,jb007mi6)
A teacher drove halfway from Point A to Point B at 120 km/h and the rest of the way at... (answered by pv=nrt15rcks)
1. pipe A can fill a tank in 5 h. pipe B can fill it in 2 h less time than it takes pipe... (answered by ptaylor)
Donna drove half the distance of a trip at 40 mi/h. At what speed would she have to drive (answered by oberobic,ankor@dixie-net.com)
Donna drove half the distance of a trip at 40 mi/h. At what speed would she have to drive (answered by robertb)
sarah drove for 7 hours at an average speed of 55 mi/h. for the first 3 hours, she drove... (answered by rwm)
evan drove halfway home at 20 miles per hour then sped up and drove the rest of the way... (answered by ankor@dixie-net.com)
Chris's average speed for a 6 hour trip was 70mi/h. for the first 4 hours, Chris drove at (answered by ankor@dixie-net.com)
Elizabeth drove the first half of a trip at 36 mi/h. At what speed should she cover the... (answered by checkley75)