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

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