Question 1081494
Your friend flew to Miami in a plane that averaged 600 mi/h.
 She drove back home, averaging 50 mi/h.
 What is her average speed in miles per hour for the whole trip, rounded to the nearest tenth?
 Assume the distances traveled by the plane and car are equal.
:
let d = the distance of the plane and the distance of the car
let s = average speed of the trip

Write time equation, time = dist/speed
:
Plane time + car time = total time
{{{d/600}}} + {{{d/50}}} = {{{(2d)/s}}}
multiply by 600s, cancel the denominators and you have:
ds + 12ds = 600(2d)
13ds = 1200d
divide both sides by d
13s = 1200
s = 1200/13
s = 92.3 mph is the average speed
:
:
Confirm this, find actual times if the distance is 600 mi
600/600 = 1 hr
600/50 = 12 hrs
----------------
1200/92.3 ~ 13 hrs 
;
Find the times if the distance is 300 mi
300/600 = .5 hrs
300/50 = 6.0 hrs
-------------------
600/ 92.3 ~ 6.5 hrs 
Just to prove that the distance really does not matter