Final answer:
The projectile takes 20 seconds to reach its maximum height.
Step-by-step explanation:
In order to find the time it takes for the projectile to reach its maximum height, we need to determine when the vertical velocity becomes zero. We can do this by finding the t-value that makes the derivative of the height function, h'(t), equal to zero. The height function is given by h(t) = -16t^2 + 640t. Taking the derivative and setting it equal to zero, we have:
-32t + 640 = 0
t = 20 seconds
Therefore, the projectile takes 20 seconds to reach its maximum height.