On #19 of http://www.allaboutcircuits.com/worksheets/dc_s.html I can't figure out how to get P =321.1W. Seems like I need to figure out the resistance value of "the water heater" using 110V and 500W. Using P=Esquared/R I come up with .041 and that's wrong. Please help. If you know of anywhere there are expanded answers to the worksheets I'd appreciate the link.