<PRE><B>Question 40797</B>
r = rate of boat in still water
t = time for both trips
For upstream
9 = (r-3)t
for downstream
11 = (r+3)t
solve each for r
r-3 = 9/t
r = 9/t + 3
--------------
r+3 = 11/t
r = 11/t - 3
I&#39;ll plot these equations, t on the horizontal axis, r on the vertical.
{{{ graph( 500, 300, -.5, 2, -2, 32, 9/x +3, 11/x - 3) }}}
It looks like the times are equal when r is about 30 mph, the rate of
boat in still water.
check
r = 9/t + 3
r = 11/t - 3
30 = 9/t + 3
27 = 9/t
27t = 9
t = 1/3 hr
-------------
30 = 11/t - 3
33 = 11/t
33t = 11
t = 1/3 hr
OK</PRE>