Question 1078220
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<pre>
Let D be the distance between the cities.

Then the time for the trip from A to B is {{{D/60}}} hours.

     The time for the trip from B to A is {{{D/40}}} hours.


The total time for the round trip is {{{D/60 + D/40}}} = {{{(40D + 60D)/(60*40)}}} = {{{(100*D)/(60*40)}}} hours.


The length of the round trip is 2D.


Hence, the average speed is {{{(2D)/((100*D)/(60*40))}}} = {{{(2*60*40)/(100)}}} = 48 mph.
</pre>

Solved.


For the solution of similar problem with detailed explanations see the lesson

&nbsp;&nbsp;&nbsp;&nbsp;- <A /HREF=http://www.algebra.com/algebra/homework/word/travel/Calculating-an-average-speed.lesson>Calculating an average speed: a train going from A to B and back</A> 

in this site.