Answer :
The ball is on the ground whenever the height is 0:
[tex]-16t^2+64t=0\implies-16t(t-4)=0\implies t=0,t=4[/tex]
Obviously, [tex]t=0[/tex] is the time when the ball is thrown, so ignore that. It thus takes the ball [tex]t=4[/tex] seconds to hit the ground after it's been launched.
[tex]-16t^2+64t=0\implies-16t(t-4)=0\implies t=0,t=4[/tex]
Obviously, [tex]t=0[/tex] is the time when the ball is thrown, so ignore that. It thus takes the ball [tex]t=4[/tex] seconds to hit the ground after it's been launched.