0=20t-9.8t^2

Solution for 0=20t-9.8t^2 equation:

0=20t-9.8t^2

We move all terms to the left:

0-(20t-9.8t^2)=0

We add all the numbers together, and all the variables

-(20t-9.8t^2)=0

We get rid of parentheses

9.8t^2-20t=0

a = 9.8; b = -20; c = 0;
Δ = b2-4ac
Δ = -202-4·9.8·0
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*9.8}=\frac{0}{19.6}
=0
$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-20)+20}{2*9.8}=\frac{40}{19.6}
=2+0.8/19.6
$