20=-4.9t^2+60

Solution for 20=-4.9t^2+60 equation:

20=-4.9t^2+60

We move all terms to the left:

20-(-4.9t^2+60)=0

We get rid of parentheses

4.9t^2-60+20=0

We add all the numbers together, and all the variables

4.9t^2-40=0

a = 4.9; b = 0; c = -40;
Δ = b2-4ac
Δ = 02-4·4.9·(-40)
Δ = 784
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$t_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{784}=28$
$t_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(0)-28}{2*4.9}=\frac{-28}{9.8}
=-2+6/7
$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(0)+28}{2*4.9}=\frac{28}{9.8}
=2+6/7
$