Question 755383
Peter drives from his home to the store at an average speed of 40 miles per hour. 
He returns home along the same route at an average speed of 60 miles per hour.
 What is his average speed in miles per hour for the entire trip?
:
let a = his average speed for the round trip
let d = the one way distance
:
Write a time equation; time = dist/speed
:
Go time + return time = total time
{{{d/40}}} + {{{d/60}}} = {{{(2d)/a}}}
We can get rid of the denominators by mult by 120a, resulting in
3ad + 2ad = 120(2d)
5ad = 240d
divide both sides by d
5a = 240
a = 240/5
a = 48 mph is the av speed for the round trip
:
:
We can confirm this solution, assume one way is 60 mi, then we have
{{{60/40}}} + {{{6/60}}} = {{{(120)/48}}}
do the math
1.5 + 1 = 2.5