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First thing we have to do is to find the perimeter of the original rectangle having a length of 20.5 m and width 10.25 m.
We will use the formula
P = 2L + 2W
P = 2(20.5) + 2(10.25)
P = 41 + 20.5
P = 61.5 m

Since, the rectangle is cut into 2 from its center, we will just divide the perimeter of the original  rectangle by 2.
Therefore, the perimeter of the half of the rectangle = 61.5  ÷ 2 = 30.75 m