A 120 watt light-bulb is a 0.120 kilowatt light bulb.
In one hour, it uses 0.120 kilowatt-hour of energy.
If electrical energy costs $0.20 per kilowatt hour, then that light bulb costs
(0.120 kilowatt-hour / hour) x (0.20 dollar / kilowatt-hour) =
(0.120 x $0.20 / hour) =
$0.024 / hour = 2.4¢ per hour
