Chapter 5.2, Problem 32E

Energy Efficiency (Compare with Exercise 31). You are thinking of making your mansion more energy efficient by replacing some of the light bulbs with compact fluorescent bulbs and insulating part or all of your exterior walls. Each compact fluorescent light bulb costs $4 and saves you an average of $2 per year in energy costs, and each square root of wall insulation costs $1 and saves you an average of $0.20 per year in energy costs.¹⁴ Your mansion has 200 light fittings and 3,000 square feet of uniosulated exterior wall. To impress your friends, you would like to spend as much as possible but save no more than $800 per year in energy costs. (You are proud of your large utility bills.) How many compact fluorescent light bulbs and how many square feet of insulation should you purchase? How much will you save in energy costs per year?