(20+x)(80-5x)=0

Solution for (20+x)(80-5x)=0 equation:

(20+x)(80-5x)=0

We add all the numbers together, and all the variables

(x+20)(-5x+80)=0

We multiply parentheses ..

(-5x^2+80x-100x+1600)=0

We get rid of parentheses

-5x^2+80x-100x+1600=0

We add all the numbers together, and all the variables

-5x^2-20x+1600=0

a = -5; b = -20; c = +1600;
Δ = b2-4ac
Δ = -202-4·(-5)·1600
Δ = 32400
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{32400}=180$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-20)-180}{2*-5}=\frac{-160}{-10}
=+16
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-20)+180}{2*-5}=\frac{200}{-10}
=-20
$