D=20t-5t^2

Solution for D=20t-5t^2 equation:

=20D-5D^2

We move all terms to the left:

-(20D-5D^2)=0

We get rid of parentheses

5D^2-20D=0

a = 5; b = -20; c = 0;
Δ = b2-4ac
Δ = -202-4·5·0
Δ = 400
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$D_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$D_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{400}=20$
$D_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-20)-20}{2*5}=\frac{0}{10}
=0
$
$D_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-20)+20}{2*5}=\frac{40}{10}
=4
$