The perimeter of a rectangular field is equal to that of a square field. The length of the rectangular field is 30m and the width is 10m. What is the area of the square field in square meters?
一块长方形地与一块正方形地的周长相等，长方形地长30m，宽是10m，正方形地的面积是多少平方米？
<gadget id="calculator">30 + 10</gadget>
<output>40</output>

<gadget id="calculator">40 * 2</gadget>
<output>80</output>

<gadget id="calculator">80 / 4</gadget>
<output>20</output>

<gadget id="calculator">20 * 20</gadget>
<output>400</output>

<result>400</result>