Question 1116608
r, boat speed if no stream current

<pre>
           SPEED       TIME             DISTANCE

UP          r-5        {{{9/(r-5)}}}           9

DOWN        r+5        {{{9/(r+5)}}}           9

TOTAL                  3
</pre>

{{{highlight_green(9/(r-5)+9/(r+5)=3)}}}

-

{{{3/(r-5)+3/(r+5)=1}}}


{{{3(r+5)+3(r-5)=r^2-25}}}

{{{6r=r^2-25}}}

{{{r^2-25-6r=0}}}

{{{r^2-6r-25=0}}}

--

discriminant  {{{36+4*25=136=4*34}}}


{{{r=(6+2*sqrt(34))/2}}}

{{{highlight(r=3+sqrt(34))}}}


8.8 km/hour