$(4)$ The beacon on a lighthouse makes one revolution every $20$ seconds. The beacon is $300$ feet from the nearest point, $\backslash ($ P $\backslash ),$ on a straight shoreline. Find the rate at which the ray of light moves along the shore at a point $200$ feet from $\backslash ($ P $\backslash ).($In other words, if you were driving in as car ulong the shoreline so that you keep the light constantly on the car, how fast would you be going when you are $200$ fout from the point on the shore nearest the light?$)$