This is College Physics Answers with Shaun Dychko. We're going to calculate the cost of running a 15-kilowatt air conditioner for 30 days only 3 hours per day where the cost of electricity is 0.110 cents per kWh. So we set up our factors with the units in mind. So we want cost and so we start by writing down dollars per kWh and these are the units we want in our numerator. So we have 0.110 dollars for every kWh and this denominator tells us what things we need to follow: we need to have something with units of hours to cancel with these hours and we need something with units of kilowatts to cancel with these kilowatts. So we have 3 hours per day now the hours cancel but we are left with these days in the denominator so we multiply that by 30 days so the days cancel and then we multiply by 15 kilowatts and then kilowatts cancel leaving us with dollars and this works out to 149 dollars.