Fold the largest square out of a piece of A4 paper with a length of 30 cm and a width of 20 cm. What is the area of this square?
从一张长30厘米、宽20厘米的a4纸上折出一个最大的正方形，这个正方形的面积有多大？
<gadget id="calculator">20 * 20</gadget>
<output>400</output>

<result>400</result>