Question 440712
Okay so 20 feet perimeter means that each side is 5 feet long. To find the diagonal, take two sides and apply the Pythagoras Theorem. So, 
 5^2+5^2=diagonal^2
 25+25=diagonal^2
 50   =diagonal^2
Taking square root on both sides, we get:
 7.071=diagonal

Since you want the answer to the nearest foot, you get 7 feet.  :)