Question 1151347
.


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, &nbsp;and learn from there on how to solve such problems.



There is a general formula for the average speed of the forth and back trip


    &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;{{{V[average]}}} = {{{(2*V[1]*V[2])/(V[1]+V[2])}}},


which you can learn from this lesson, also.



By substituting the given data into the formula, you get the answer momentarily


    &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;{{{V[average]}}} = {{{(2*100*80)/(100+80)}}} = {{{16000/180}}} = {{{1600/18}}} = {{{800/9}}} km/h.


consistent with the other tutors.



I do not convince you to memorize this formula.


But every student, who is competent in Math or who wants to be considered as competent in Math, 
must know that such formula does exist and should be able to deduce and to apply it (!)



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How @MathLover1 treats this problem, it is a classic example on how this problem &nbsp;<U>SHOULD &nbsp;NOT</U> &nbsp;be treated.


Every textbook and every teacher explains it to their students with all needed warnings (!) (!) (!)