A boat is pulled into a dock by means of a rope attached to a pulley on the dock (see the accompanying figure). The rope is attached to the bow of the boat at a point 20 ft below the pulley. If the rope is pulled through the pulley at a rate of 20 ft/min, at what rate will the boat be approaching the dock when 100 ft of rope is out?

Enter absolute value of the rate as an improper fraction if needed.
The boat is approaching the dock at the rate of ???