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)
