A hot air balloon rising vertically is tracked by an observer located 4 km from the lift-off point. At a certain moment, the angle between the observer's line of sight and the horizontal is, and it is changing at a rate of 0.1 rad/min. How fast is the balloon rising at this moment? Let y be the height of the balloon (in km), t be time (in minutes), and the angle between the line-of-sight and the horizontal (in radians). (Use decimal notation. Give your answer to two decimal places.) km/min 22