Question 1144577
<pre>
              SPEED         TIME        DISTANCE

firstpart     r             84/r         84

secondpart    r+4          84/(r+4)      84

difference                 1/2
</pre>
{{{84/r-84/(r+4)=1/2}}}

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{{{r^2+4r-672=0}}}

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{{{highlight(r=24)}}}