A Professor of Economics at University of Joy earns $200 an hour as a Professor.She can hire a Housekeeper for $15 an hour to take care of her home.The Professor is very smart and hard working.She has found out that she can do the Housekeeping work twice as fast as her Housekeeper.Should she fire the Housekeeper and do the work herself?What principle in Economics should be used in answering this question?