200/(x+20)=200/x-1/2
1/(x+20)=(200/x-1/2)/200
(x+20)*(200/x-1/2)=200
(x+20)*(400-x)=200*2x
400x-x^2+8000-20x=400x
-x^2-20x+8000=0
x^2+20x-8000=0
x1=-100 x2=80