On a certain island, the train Clarabel is stuck. Thankfully, Clarabel's track intersects Bertram's track (at switch D), and there is a 50 foot steel cable welded to Bertram and Clarabel. Now Bertram is also stuck, but Bertram's track intersects Arthur's track (at switch E), and there is a 50 foot iron chain welded to Bertram and Arthur.
Arthur is travelling away from switch E at a speed of 10 feet per minute, and is currently 30 feet from switch E. Bertram is 40 feet from switch E and 30 feet from switch D. Clarabel is currently 40 feet from switch D.
Be “Really Useful” by computing how fast Clarabel is moving toward switch D.