Question 957182
ship speed in still water, 25 mph
<pre>  
                      SPEED         TIME            DISTANE
with strean           25+s           t              256
against stream        25-s           t              144
</pre>
{{{system(t=256/(25+s), and, t=144/(25-s))}}}


{{{256/(25+s)=144/(25-s)}}}


{{{16/(25+s)=9/(25-s)}}}


{{{16(25-s)=9(25+s)}}}

{{{400-16s=225+9s}}}

{{{175=25s}}}

{{{highlight(s=7)}}}