Question 1207221
<pre>
               SPEED          TIME       DISTANCE(kilometers)

GOING TO         r+c            4           d

RETURNING        r-c            5           d

condition        r+c            3.5          70
</pre>
r, speed of boat without current
c, speed of the current


{{{system(4(r+c)=5(r-c),r+c=70/3.5)}}}



See a very obvious substitution, for r+c.

{{{system(4(70/3.5)=5r-5c,r+c=70/3.5)}}}


Simpler,  {{{system(5r-5c=80,r+c=20)}}}


Even still simpler
{{{system(r-c=16,r+c=20)}}}