Question 1154291
U=156 kilometers
D=252 kilometers
T=6 hours
r, unknown boat speed in still water
c, unknown speed of the stream

<pre>
            SPEED       TIME       DISTANCE

UP             r-c       T            U

DOWN           r+c       T            D
</pre>
{{{system(r-c=U/T,r+c=D/T)}}}

-

{{{ADD}}}:
{{{2r=(U+D)/T}}}

{{{highlight(r=(U+D)/(2T))}}}



{{{SUBTRACT}}}:
{{{2c=(D-U)/T}}}

{{{highlight(c=(D-U)/(2T))}}}