Question 971783
You missed a datum in the description.  Not too bad an omission; I will generalize.


r, speed of boat for still water, UNKNOWN
c, speed of river current
d, distance going downstream
u, distance going upstream
t, time to travel either direction, UNKNOWN



___________________rate___________time__________distance
UPSTRM_____________r-c____________t_____________u
DOWNSTR____________r+c____________t_____________d


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


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


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


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


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


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


{{{dr-ur=dc+uc}}}, which makes sense for the signs


{{{r(d-u)=dc+uc}}}


{{{highlight(r=(dc+uc)/(d-u))}}}