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Imagine that you find yourself standing in front of a large billboard. The bottom and top edges of the billboard are located $5$ and $15\text{ ft}$ above your eye level, respectively. As you walk directly away from where the billboard is mounted at a speed of $4\text{ ft/s}$, you continually have the billboard in your line of vision. Determine how quickly the subtended angle $\theta$ by the billboard from your eyes is changing when you are located $25\text{ ft}$ away from where the billboard is mounted. Assume the ground you are walking on is flat.