Question 1017321
RT=D travel rates rule


<pre>
                   rate       time        distance
Down               40+c        4           d
Up                 40-c        10          d
</pre>


RT=D
R=D/T
{{{system(40+c=d/4,40-c=d/10)}}}


OR much simpler, {{{(40+c)*4=(40-c)*10}}}.
Solve this for c.