If you draw a line parallel to the side of the cone through the center of the 10-inch (radius) circle, you create a right triangle with one leg 10 inches and a hypotenuse of 20+10 = 30 inches. It is similar to the right triangle shown on the diagram with leg 10 inches. That means the distance from the center of the 10-inch (radius) ball to the bottom of the cone is 30 inches.
Subtract from that the 10-inch radius and the 10-inch diameter of the two bottom balls shown, and you find the distance from the base of the smallest ball to the bottom of the cone is 30 -10 -10 = 10 inches.
