Question 1108263
.
<pre>
Your "time" equation to find the distance d is


{{{d/20}}} + {{{d/18}}} = 8   hours.


Each term (each ratio) in the left side is the time to cover the one way distance.


Multiplying both sides by 180 (common denominator) you get


9d + 10d = 8*180


19d = 1440  ====>  d = {{{1440/19}}} = 75.789 miles (approx.)
</pre>-------------


<U>Be aware</U> :  The basic equation by @josgarithmetic is written incorrectly !