When linear measures are changed by a factor of k, area changes by a factor of k^2.
The area of EFGH will be multiplied by 4 if its perimeter is doubled.
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Original perimeter = s + s + s + s = 4s. Original area = s^2.
New perimeter = 2s + 2s + 2s + 2s = 8s. New area = (2s)^2 = 4s^2, 4 times the original area.