A120-watt light bulb uses about 0.1 kilowatt of electricity per hour. if electricity costs $0.20 per kilowatt hour, how much does it cost to have the bulb on for an hour?

There is no such thing as a "kilowatt per hour". If that's actually what the question says, then it's a defective question, and you should put it away before it makes you any more confused. A 120 watt light bulb uses exactly 0.12 kilowatt when it's turned on. In one hour, it uses (0.12 kilowatt) x (1 hour) = 0.12 kilowatt-hour of energy. If energy costs $0.20 per kilowatt-hour, then the cost is (0.12) x (0.20) = 2.4 cents. (0.024 dollar)

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