Occasionally, stones in the gallbladder (gallstones) and stones in the kidney (kidney stones) are treated with a procedure called lithotripsy, by which a shock wave is applied not at the site of the stone, but at another related point. An electro at one focus of an elliptical reflector sends high-energy shock waves to crush the kidney stone at the other focus. Suppose the length of the major axis of the ellipse is 40 centimeters and the length of the minor axis is 20 centimeters How far from the kidney stone should the electrode be placed to shatter it?