30=25t-4.9t^2

Solution for 30=25t-4.9t^2 equation:

30=25t-4.9t^2

We move all terms to the left:

30-(25t-4.9t^2)=0

We get rid of parentheses

4.9t^2-25t+30=0

a = 4.9; b = -25; c = +30;
Δ = b2-4ac
Δ = -252-4·4.9·30
Δ = 37
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{-(-25)-\sqrt{37}}{2*4.9}=\frac{25-\sqrt{37}}{9.8}
$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-25)+\sqrt{37}}{2*4.9}=\frac{25+\sqrt{37}}{9.8}
$