Question 2805
100x - 4000/100x = .3
=> (100x*100x - 4000)/100x = 3/10
=> 10000x^2 -4000 = 30x
1000x^2 -400 = 3x
1000x^2 -3x - 400 =0

Now u can find the roots of the equation by using the formulae
x = {{{(-b+-sqrt(b*b-4*a*c)/(2*a))}}}