Question 1159329
<pre>
                 SPEED       TIME        DISTANCE

IN RAIN           r          72/r           72

IF SUNNY         r+20        72/(r+20)      72

DIFFERENCE                  {{{2/5}}}
</pre>
{{{72/r-72/(r+20)=2/5}}}

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{{{r^2+20r-3600=0}}}

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{{{highlight(r=-10+10*sqrt(37))}}} about 50.83.
 and you can evaluate the time from this.