Question 1134246
R-C=100x-0.5x^2-40x-500
so -0.5x^2+60x-500=0 
solve for -0.5x^2+60x-500=500 or -0.5x^2+60x-1000=0 or x^2-120x+2000=0, multiplying the previous by -2.
(x-100)(x-20)=0, x=20 and 100
{{{graph(300,300,-30,300,-1000,2000,-0.5x^2+60x-500,500)}}}