Question 564711
you drive to the beach at a rate of 120 kilometers per hour.
 On the return trip you drive at a rate of 60 kilometers per hour.
 What is your average speed for the entire trip?
:
Let a = the average speed for the whole trip
let d = the one-way distance of the trip
:
Write time equation, time = dist/speed
:
To time + return time = total time
{{{d/120}}} + {{{d/60}}} = {{{(2d)/a}}}
Multiply by 120a
120a*{{{d/120}}} + 120a*{{{d/60}}} = 120a*{{{(2d)/a}}}
cancel the denominators
ad + 2ad = 120(2d)
3ad = 240d
Divide both sides by d
3a = 240
a = 240/3
a = 80 km/hr is the av speed for the round trip