Question 60980
When the ball hits the ground, the ball will be at zero feet in height.
0 = -16t^2 + 64t
0 = t^2 - 4t
4 = (t - 2)^2
+- 2 = t - 2
2 +- 2 = t
t = 0 and t = 4
After four seconds, the ball will hit the ground respecting your formula.