In my classroom there are 72 35-watt (W) light bulbs. How much will it cost to light my room for an entire school year if the cost of electricity is $0.10 per kilowatt-hours (kWh) ? My work year is 187 days, beginning at 7:15 am and ending at 2:45 pm.

The direct and easy solution for this problem is through dimensional analysis. This is done by cancelling out the same units of dimensions resulting to the unit of what is asked. In this case, the resulting unit should be in $.

The number of hours in a day from 7:15 am to 2:45 pm is 7.5 h. In 187 days, that would be:

187 days * 7.5 h/day = 1402.5 h

The rest of the solution goes:

1402.5 h * $0.1/kWh * 1 kW/1000 W * 35 W/bulb * 72 bulbs =

$353.43