Question 419440
Since the width of rectangular is x ft the length of this rectangle is ( 80-2x)/2
and the enclosed area is x(80-2x)/2=40x-x^2. A=40x-x^2 is an downward parabola and the value y of its vertex is the maximum area. 
 A=-x^2+40x the coordinate x=-b/2a=-40/-2=20 and the A=-20^2+40(20)=400.

Answer: The maximum enclosed area is 400 sft.