Observing a launch An observer stands 300 ft from the launch site of a hot-air balloon at an elevation equal to the elevation of the launch site. The balloon is launched vertically and maintains a constant upward velocity of 20 ft/s. What is the rate of change of the angle of elevation of the balloon when it is 400 ft from the ground? (Hint: The angle of elevation is the angle 0 between the observer's line of sight to the balloor and the ground.)