The Mac Davis Company specializes in the purchase, renovation, and resale of older homes. Mac employs several carpenters and painters to do the work for him. It is essential for him to have accurate cost estimates so he can determine total renovation costs before he purchases a piece of property. If estimated renovation costs plus the purchase price of a house are higher than the house's estimated resale value, it is not a worthwhile investment. Mac has been using the home's interior square feet for his exterior paint cost estimations. Recently he decided to include the number of external openings-the total number of doors and windows in a house-as a cost driver. Their cost is significant because they require time-consuming preparatory work and careful brushwork. The rest of the house usually is painted either by rollers or spray guns, which are relatively efficient ways to apply paint to a large area. Mac has kept careful records of these exterior painting costs on his last 12 jobs: House Square Feet External Openings 1 2,510 13 Costs $ 2,820 234567ORDIN 3,010 15 3,742 2,800 12 3,100 2,850 12 3,150 4,610 19 4,710 2,720 13 3,245 2,600 11 2,920 8 2,550 11 2,836 9 2,600 10 3,252 3,700 16 4,112 11 2,650 13 3,210 12 3,550 16 3,965