Answer:
54 feet
Step-by-step explanation:
The perimeter of the room is the sum of wall lengths:
2·(15 feet) + 2·(12 feet) = 30 feet + 24 feet = 54 feet
The border should be cut to 54 feet.
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Comment on the question
This seems to be a math problem designed to get you to figure the perimeter of the room. If you were actually installing wallpaper, you might want to cut the border in four (4) pieces, each slightly longer than the corresponding wall. That way any lack of squareness in the corners could be managed more easily, and the possibility of a gap at the seam could be eliminated.
