Question 668717
The projectile will reach its maximum height when it's velocity is zero.

The velocity is the derivative of the position with respect to time.

In this case, position is denoted by h and given as:

h=-13t^2+520t

Differentiate h with respect to t to obtain the velocity:

v=dh/dt=-26t+520

Now set v equal to zero and solve for t:

-26t+520=0

t=20 sec(Answer)