A fortune teller's "crystal ball" (actually just glass) is 13 cm in diameter. Her secret ring is placed 8 cm from the edge of the ball.
a) An image of the ring appears on the opposite side of the crystal ball. How far is the image from the center of the ball?
b) The crystal ball is removed and a thin lens is placed where the center of the ball had been. If the image is still in the same position, what is the focal length of the lens?