30=-4.9t^2+15t+20

Solution for 30=-4.9t^2+15t+20 equation:

30=-4.9t^2+15t+20

We move all terms to the left:

30-(-4.9t^2+15t+20)=0

We get rid of parentheses

4.9t^2-15t-20+30=0

We add all the numbers together, and all the variables

4.9t^2-15t+10=0

a = 4.9; b = -15; c = +10;
Δ = b2-4ac
Δ = -152-4·4.9·10
Δ = 29
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$t_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$t_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-15)-\sqrt{29}}{2*4.9}=\frac{15-\sqrt{29}}{9.8}
$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-15)+\sqrt{29}}{2*4.9}=\frac{15+\sqrt{29}}{9.8}
$