Question 1166481
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From the condition, you have this "time" equation


    {{{163/(x+15)}}} - {{{120/x}}} = {{{3/60}}},  or


    {{{163/(x+15)}}} - {{{120/x}}} = {{{1/20}}}.


where x is the average speed under the question, in miles per hour.


To find x, first multiply both sides by  20x*(x+15);  reduce it to the standard form quadratic equation and then solve it.
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