Question 1129852
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Let r be the usual speed.

Then you have this "time equation"


{{{140/r}}} - {{{140/(r+5)}}} = 0.5    of an hour.


280*(r+5) - 280*r = r*(r+5)


r^2 + 5r - 1400 = 0


(r+40)*(r-35) = 0


The only meaningful is the positive root  r = 35 miles per hour.    <U>ANSWER</U>


<U>Check</U>.   {{{140/35}}} - {{{140/(35+5)}}} = 4 - 3.5 = 0.5  of an hour.   ! Correct !
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Solved.