3. (13 points) A spherical soap bubble has radius r and thickness h. The volume of soap fluid that forms the bubble is related to the radius and thickness by the formula: 3 4 V = - - - [(- + 1) - (- -)] r- 3 Suppose that the bubble is expanding but the total volume of soap fluid remains constant. At a certain moment, the radius is 20 cm and increasing at 0.5 cm/sec. At the same time, the thickness is 2 cm. How fast is the thickness decreasing at this moment?