Question 1180023
<pre>
            SPEED      TIME                 DIST.

DOWN         15+c      84/(c+15)              84

UP           15-c      18/(15-c)              18
</pre>

{{{84/(15+c)=2(18/(15-c))}}}



{{{7/(15+c)=3/(15-c)}}}

{{{7(15-c)=3(15+c)}}}

{{{7*15-7c=3*15+3c}}}

{{{4*15=10c}}}

{{{c=60/10}}}

{{{highlight(c=6)}}}