There is a square vegetable field with a length of 30 meters. From it, a rectangular land with a length of 30 meters and a width of 10 meters is drawn to plant cucumbers. The rest is planted with potatoes. How many square meters is the area for planting potatoes?
有一块长30米的正方形菜地，从里面划出一块长30米、宽10米的长方形土地种黄瓜，剩下的种土豆，种土豆的面积是多少平方米？
<gadget id="calculator">30 - 10</gadget>
<output>20</output>

<gadget id="calculator">30 * 20</gadget>
<output>600</output>

<result>600</result>