Christopher is going to install wallpaper on the four walls of his room. two of the walls are 14 feet long by 9 feet high. the other two walls are 20 feet long by 9 feet high. his options at the wallpaper store are below. one single roll of wallpaper costs $20 and covers 18 square feet of area. one double roll of wallpaper costs $40 and covers 54 square feet of area. what is the minimum cost of installing the wallpaper on the four walls of christopher's room?

Given:
2 walls - 14ft by 9ft
2 walls - 20ft by 9ft

Wall 1 & 2 = 14ft * 9ft = 126 ft² x 2 = 252 ft²
Wall 3 & 4 = 20ft * 9ft = 180 ft² x 2 = 360 ft²

Total area = 252 ft² + 360 ft² = 612 ft²

Cost of 1 single roll: $20
612 ft² ÷ 18 ft² = 34 rolls * $20 = $680

Cost of double roll: $40
612 ft² ÷ 54 ft² = 11.33 rolls ⇒ 12 double rolls
12 * $40 = $480

If Christopher opts for 1 single roll of wallpaper, he'll spend $680.
If he opts for double roll of wallpaper, he'll spend $480.

Double roll of wallpaper is the cheaper option. He'll save $200 if he'll buy the double roll of wallpaper.