Question 1056020
{{{system(r=2,d=14,u=7,t=unknownTimeBothDirections,c=unknownRiverCurrentSpeed)}}}

<pre>
                  SPEED      TIME     DISTANCE
DOWNSTREAM         r+c        t        d
UPSTREAM           r-c        t        u
</pre>

{{{system(t=d/(r+c),and,t=u/(r-c))}}}


{{{d/(r+c)=u/(r-c)}}}


{{{d(r-c)=u(r+c)}}}


{{{dr-dc=ur+uc}}}


{{{dr-ur=uc+dc}}}


{{{dr-ur=(u+d)c}}}


{{{highlight((dr-ur)/(u+d)=c)}}}