Question

A projectile is thrown upward so that its distance above the ground after t seconds is given by the function h(t) = -16t2 + 640t. After how many seconds does the projectile take to reach its maximum height?

Answer 1

It will reach maximum height when 
h'(t) = -32t + 640 = 0 
-32t = -640 
t = -640/-32 = 20 seconds