Because the diagonal of a square is vertical, it can be calculated by the formula that the area of a quadrilateral with orthogonal diagonal is half of the product of two diagonal lines.

The diagonal length of a small square is the diameter of the inscribed circle of a large square, that is, the side length of the large square.

So the area of the small square is half of the product of the side length of the large plastic, which is half of the area of the large square, which is 30.