0=20t-0.81t^2

Solution for 0=20t-0.81t^2 equation:

0=20t-0.81t^2

We move all terms to the left:

0-(20t-0.81t^2)=0

We add all the numbers together, and all the variables

-(20t-0.81t^2)=0

We get rid of parentheses

0.81t^2-20t=0

a = 0.81; b = -20; c = 0;
Δ = b2-4ac
Δ = -202-4·0.81·0
Δ = 400
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$t_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{400}=20$
$t_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-20)-20}{2*0.81}=\frac{0}{1.62}
=0
$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-20)+20}{2*0.81}=\frac{40}{1.62}
=24+1/1.4464285714286
$