Question 761779
A bicyclist riding against the wind averages 10 miles per hour in
 traveling from A to B. 
With the wind, averages 15 miles per hour in returning from B to A,
 how many miles per hour is his average speed for the round trip?
:
Let a = average speed for the trip
Let d = the one-way distance
:
Write a time equation: time = dist/speed
:
against time + with time = total time
{{{d/15}}} + {{{d/10}}} = {{{(2d)/a}}}
multiply by 30a to clear the denominators, results
2a(d) + 3a(d) = 30(2d)
2ad + 3ad = 60d
5ad = 60d
divide both sides by d
5a = 60
a = 60/5
a = 12 mph is the average speed for the trip