.
There is ANOTHER way to solve the problem, and it's worth to know it.
Let d be one way distance.
Then the time to go "there" is hours.
The time to go "back" is hours.
The total time is 1 hour, which gives you an equation
+ = 1 hour.
To solve it, multiply both sides by 15. You will get
5d + 3d = 15, or 8d = 15, d = .
So, one way distance is miles.
Hence, the total distance to "there" and "back" is twice miles, or mile = miles. ANSWER
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