Question 930733
 Genny travels from one elementary school to another and back at a rate of 20 miles per hour one way and 30 miles per hour another way.
 What is her average rate of speed for both trips?
:
let d = the one way distance of the trip
let a = the average speed for the round trip
:
Write a time equation; time = dist/speed
to time + return time = total time
{{{d/20}}} + {{{d/30}}} = {{{(2d)/a}}}
multiply equation by 60a; cancel the denominators
3ad + 2ad = 60(2d)
5ad = 120d
divide both sides by d
5a = 120
a = 120/5
a = 24 mph is the average for the round trip