Question 1134632
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There is ANOTHER way to solve the problem, and it's worth to know it.


<pre>
Let  d  be one way distance.


Then the time to go "there" is  {{{d/3}}}  hours.


The time to go "back"  is  {{{d/5}}}  hours.


The total time is 1 hour, which gives you an equation


{{{d/3}}} + {{{d/5}}} = 1    hour.


To solve it, multiply both sides by 15. You will get


5d + 3d = 15,   or   8d = 15,   d = {{{15/8}}}.


So, one way distance is  {{{15/8}}}   miles.


Hence, the total distance to "there" and "back"  is  twice {{{15/8}}} miles,  or  {{{15/4}}} mile = {{{3}}}{{{3/4}}} miles.    <U>ANSWER</U>
</pre>

Solved.


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As a comment to the @Alan3354's post, the Michelson-Morley experiment was performed for the first time in the year 1887 - 
it is NOT the mid of the 17-th century; it is at the end of the 19-th century.


See this Wikipedia article 
https://en.wikipedia.org/wiki/Michelson%E2%80%93Morley_experiment