39. The changing quantities a and b are related by the equation a3b2 = 432. Find a when a = 3, b = 4 and - = 2 40. The volume of a spherical balloon of radius r is - . Find the rate at which the radius is changing when the balloon has a volume of 100 cubic inches and the volume is increasing at the rate of 2 cubic inches per second. 41. The sides of a cube are expanding at a rate of 2 inches per minute. How fast is the volume of the cube changing at the moment when the area of the base of the cube is 10 square inches? Give your answer in correct units. 42. The top of a ladder slides down a vertical wall at a rate of 0.15 m/s. At the moment when the bottom of the ladder is 1.5 m from the wall, it slides away at a rate of 0.2 m/s. How long is the ladder? 43. After a rupture, oil begins to escape from an underwater well. As the oil rises, it forms a circular slick whose radius increases at a rate of 0.42 ft/min. Find the rate at which the area of the spill is increasing when the radius is 120 ft. State your answer in proper units. 44. A rocket travels vertically at a speed of 1200 km/h. The rocket is tracked through a telescope by an observer located 16 km from the launching pad. Find the rate at which the angle between the telescope and the ground is increasing 3 minutes after lift-off