What is the minimum sample size needed to be 80% confident with a 3.9% margin of error?

See below in bold.

Step-by-step explanation:

a)  h = -3t^2 + 15t = 0

-3t(t - 5) = 0

t = 0, 5 are the zeroes of the relation.

b)  The ball hits the ground when h = 0 so it is after 5 seconds.

c)  The path of the ball is a parabola which opens downwards so the maximum height  is when x = 5/2 = at 2.5 seconds.

d) this maximum height is -3(2.5)^2 + 15(2.5) = 18.75 meters.