(Solved): 4. (12 points) A hot-air balloon is rising vertically at the rate of 5 meters ...

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4. (12 points) A hot-air balloon is rising vertically at the rate of 5 meters per second. An observer is standing on the ground 300 meters from where the balloon was released. At what rate is the distance between the observer and the balloon changing when the balloon is 400 meters high? [Hint: The stationary observer is not moving anywhere. That is, the rate of change of the distance from the observer to the spot the balloon was released on the ground is 0 meters per second.]