Question 1125519
x, boat speed when no current
y, speed of current


{{{system(x+y=210/10,x-y=210/70)}}}


{{{system(x+y=21,x-y=3)}}}


{{{2x=24}}}
{{{x=12}}}
-
{{{x+y-x-(-y)=21-3}}}
{{{2y=18}}}
{{{y=9}}}
...


{{{x=12}}}------boat speed without current
{{{y=9}}}------speed of the current