Question 1157514
x, x, 80-2x
The three lengths
80-2x is length parallel to riverbank


{{{x(80-2x)}}}, area


Maximum area should be in exact middle between the zeros.

One zero is 0.

Other zero is {{{80-2x=0}}}
{{{40-x=0}}}
{{{x=40}}}


Exact middle of 0 and 40 is 20.


Dimensions to max area:

20 and 40.