Question 1081031
y=80-2x
substitute 
A=x(80-2x)=80x-2x^2
derivative is 80-4x
set that equal to 0 and 80=4x
x=20
y=40
That adds to 80 and the product is 800.
Try x=19 and y=42, product is 798
try x=21 and y=38, product is 798
{{{graph(300,300,-10,30,-50,850,-2x+80,80x-2x^2)}}}