20=-16t^2+40t+1.5

Solution for 20=-16t^2+40t+1.5 equation:

20=-16t^2+40t+1.5

We move all terms to the left:

20-(-16t^2+40t+1.5)=0

We get rid of parentheses

16t^2-40t-1.5+20=0

We add all the numbers together, and all the variables

16t^2-40t+18.5=0

a = 16; b = -40; c = +18.5;
Δ = b2-4ac
Δ = -402-4·16·18.5
Δ = 416
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$t_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

The end solution:

$\sqrt{\Delta}=\sqrt{416}=\sqrt{16*26}=\sqrt{16}*\sqrt{26}=4\sqrt{26}$
$t_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-40)-4\sqrt{26}}{2*16}=\frac{40-4\sqrt{26}}{32}
$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-40)+4\sqrt{26}}{2*16}=\frac{40+4\sqrt{26}}{32}
$