Suppose that you and your roommate are sharing the electric bill in your apartment and that it comes to $60/month, at the average rate of $0.1975 per kilowatt-hour. Your roommate never turns the lights off, which you estimate results in the lights being switched on for an average of 19 hours per day - 6 hours more than needed. The room is lit with seven, 60 W incandescent bulbs, which you can assume are powered by a 115 V constant voltage source.
How much does lighting cost you and your roommate per month?