Maximum height: 20 m
Initial velocity: 20 m/s
When an object is thrown straight up, meaning that its angle of launch forms a
Since you know tha it takes 4 seconds for the ball to return to the thrower, it follows that
To determine the initial velocity of the ball, use the fact that, at maximum height, the velocity of the ball is equal to zero.
This implies that the ball was thrown with an initial velocity of
To get the maximum height of the ball, use the equation that describes its vertical displacement