It would take 1.25 seconds.
The equation would be [tex]0=-16t^2+8t+15[/tex], since the original velocity is 8 and the original height is 15. Using the quadratic formula we have
[tex]t=\frac{-8\pm \sqrt{8^2-4(-16)(15)}}{2(-16)}
\\
\\=\frac{-8\pm \sqrt{64--960}}{-32}=\frac{-8\pm \sqrt{1024}}{-32}
\\
\\=\frac{-8\pm 32}{-32}=\frac{-8+32}{-32}\text{ or } \frac{-8-32}{-32}
\\
\\=\frac{24}{-32}\text{ or }\frac{-40}{-32}=-0.75\text{ or }1.25[/tex]
Since negative time makes no sense, the time is 1.25 seconds.
