The area of the path is the difference between the areas of the outer rectangle and the inner rectangle.
Outer rectangle dimensions: 35 x 20, area=700 sq m.
Inner rectangle dimensions: 31 x 16, area=496 sq m., because the length and width are shortened by twice the width of the path.
Area of path=700-496=204 sq m.
Cost=204 x 25=Rs5100.