Low-energy lightbulbs typically cost $3.50, have a life of 9 years, and use about $1.60 of electricity a year. Conventional lightbulbs are cheaper to buy, for they cost only $.50. On the other hand, they last only about a year and use about $6.60 of energy. If the discount rate is 5%, which product is cheaper to use?

Solve by hand, not in excel please.