Kate needs to lift a 500 N box of books onto a library shelf that is 3 meters high. She decides that the best machine to help her would be a set of fixed and movable pulleys known as a block and tackle. She applies an effort force of 100 N to the rope of the pulley and finds that she has to pull 18 meters of rope to get the box raised to the height of the shelf. What is the efficiency of the pulley system?