Question 1153860
<pre>
                   SPEED            TIME           DIST.

M to K            8+c               12/(8+c)       12

K to M            10-c              12/(10-c)      12

                             {{{12/(8+c)=12/(10-c)}}}
</pre>
Solve the equation for c.




If you have more examples just like this, then using G for the downstream boat and L for the upstream boat, {{{G>L}}},  D for distance each way, c for unknown speed of the river, then:
<pre>
           SPEED      TIME     DIST.
MtoK       L+c        D/(L+c)   D
KtoM       G-c        D/(G-c)   D
</pre>
{{{D/(G-c)=D/(L+c)}}}
-
{{{1/(G-c)=1/(L+c)}}}

{{{G-c=L+c}}}

{{{G-L=2c}}}

{{{highlight_green(c=(G-L)/2)}}}