Question 1016037
A couple of ways to do this, but the simplest may be to solve this equation for when h = 0.
Obviously h = 0 at t = 0, but let's solve for t...
-16t^2 + 64t = 0
-t^2 + 4t = 0
t(4 - t) = 0
t = 0, 4
Time is 4 seconds in the air.