A balloon rises at a rate of 4 meters per second from a point on the ground 50 meters from an observer.

A balloon rises at a rate of 4 meters per second from a point on the ground 50 meters from an observer. Find the rate of change of the angle of elevation of the balloon from the observer when the balloon is 50 meters above the ground. I know that the answer to this question is 1/25 radians per second but how do you work it out to get this answer?