Question 448374
40x20=800
(40+2x)(20+2x)=1600
800+120x+4x2=1600
4x2+120x-800=0
x2+30x-200=0
*[invoke quadratic "x", 1, 30, -200 ]
Throwing out the negative answer, we get x=5.61552812, which is the amount to be added to all 4 sides..