Answer: 3600 BTUs
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Work Shown:
Area of rectangle = length*width
A = L*W
A = 15*12
A = 180
Sue's bedroom has floorspace of 180 square feet.
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1 square foot = 20 BTUs required
180*(1 sq ft) = 180*(20 BTUs) ... multiply both sides by 180
180 square feet = 3600 BTUs required