Question 404876
A square has 4 equal sides.  Let's use x to represent the length of a side.  
The perimeter is the distance around the square, and since there are 4 sides, the perimeter of the square would be 4x.  Let's set up an equation and solve for x.
80=4x
/4 /4  Division property of equality
20=x 

The area of a square is side squared, or x².  We know that x=20 so we can substitute and solve for the largest area.
20²=400.  Thus the answer is 400 cm squared.