Question 145012
if height of the object is given as a function of time so its derivative gives velocity.as ball is thrown upward so its final velocity will be zero or dh/dt=0
differentiate h=64t-16t^2 w.r.t 
dh/dt= 64-32t  put dh/dt=0
64-32t=0
t=2 seconds.ball takes 2 seconds to attend maximum height and 2 seconds to return so total 4 seconds it remains in the air