Teacher Zhang needs to cut out the largest square from a piece of cardboard with a length of 30 cm and a width of 20 cm. How many square centimeters is the rest?
张老师要从一张长30厘米，宽20厘米的卡纸上剪下一个最大的正方形．剩下部分是多少平方厘米？
<gadget id="calculator">30 * 20</gadget>
<output>600</output>

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

<gadget id="calculator">600 - 400</gadget>
<output>200</output>

<result>200</result>