You want to wallpaper a room. your room is 10 feet by 12 feet and the ceiling height is 10 feet. a double roll of wallpaper covers approximately 56 square feet of wall. how many double rolls of wallpaper should you buy?

9 double rolls

6 double rolls

8 double rolls

7 double rolls

What you need to calculate is surface area of the room.
SA = 12 * 10 * 2 (for the two 12 ft long walls) = 240 ft^2
SA = 10 * 10 * 2 (for the two 10 ft long walls = 200 ft^2
200 + 240 = 440 total square feet of wall space.
Each double roll covers 56 sq ft.
440/56 = 7.86 double rolls; you cannot buy a partial double roll so we need to buy 8 double rolls