A window washer on a hanging platform cleans the outside of windows on a skyscraper. The washer hoists the platform up the side of the building by pulling down on a rope that runs up over a pulley and then runs back down and connects to the top of the platform. The combined mass of the washer and the platform is 100kg, and the washer pulls on the rope with a force of 800N. What is the acceleration of the platform?