Question 1157811
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Let x be his average speed.

Then the other speed is (x+4) mph.


From the condition, you have this equation


    {{{175/x}}} = {{{189/(x+4)}}}.


Cross multiply, simplify and solve for x


    175*(x+4) = 189x

    175x + 4*175 = 189x

    4*175 = 189x - 175x

    4*175 = 14x

    x     = {{{(4*175)/14}}} = 50.


<U>ANSWER</U>. The average speed was 50 miles per hour.
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Solved.