Low$-$energy light bulbs typically cost $$3\mathrm{.}60,$ have a life of $9$ years, and use about $$2\mathrm{.}00$ in electricity a

year. Conventional light bulbs are less expensive, since they cost only $$0\mathrm{.}60,$ but they last only a year

and use about $$7\mathrm{.}00$ in electricity. If the discount rate is $4\%,$ which product is cheaper to use? $($hint:

first determine the NPV of the costs associated with each type of light bulb, and then convert this NPV

into an equivalent annuity, or an equivalent annual cost$).$