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The barn is represented by the rectangle ABCD in the figure; AB = CD = 20 ft and BC = DA = 10 ft.
The cow is tied to the corner with a 50 ft rope.
So it can graze over the area of the circle shown (centre of circle at corner point A and radius of circle is 50 ft).
It is evident from the figure that the cow will be able to graze over the entire barn (area = 20 x 10 = 200 sq ft).