Final answer:
To find the initial speed of the ball, separate the horizontal and vertical motions, calculate the time to fall using the height and gravity, and then determine the horizontal speed with the distance and time.
Step-by-step explanation:
The question involves a ball thrown horizontally from a height of 20 meters that lands 15 meters away. To determine the initial speed at which it was thrown, we use the kinematic equations for projectile motion. Since there is negligible air resistance, the horizontal and vertical motions can be considered separately. The formula for the time it takes for the ball to hit the ground, derived from vertical motion, is t = √(2h / g), where h is the height and g is the acceleration due to gravity (9.81 m/s2). The horizontal speed v is then found using the formula v = d / t, where d is the horizontal distance traveled.