Answer: 30 feet by 55 feet
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The blueprints say that 1 inch represents 10 feet. This means we can form the equation below
1 inch = 10 feet
Of course 10 feet is much larger than 1 inch, so they cannot be actually equal to one another, but it helps tie together the two quantities. And it also allows us to multiply both sides by 3 to get
3 inches = 30 feet
So 3 inches on paper means we have 30 feet in real life.
Similarly,
1 inch = 10 feet
5.5 inches = 55 feet (multiply both sides by 5.5)
The 3 inch by 5.5 inch rectangle on paper corresponds to a 30 feet by 55 feet rectangle in real life.
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In short, you multiply each given dimension by 10
3 inches goes to 3*30 = 30 feet
5.5 inches goes to 5.5*10 = 55 feet