A cuboid iron sheet (thickness is not counted), the four corners are cut off to make a square with a side length of 10 cm, and welded into a cuboid iron sheet box without a cover, which can hold 3 liters of oil. Given that the length of this rectangular iron sheet is 40 cm, find the area of the rectangular iron sheet.
一块长方体铁皮(厚度不计)，四个角剪去边长为10厘米的正方形，焊成一个无盖的长方体铁皮盒可以盛油3升．已知这块长方形铁皮的长为40厘米，求长方形铁皮的面积．
<gadget id="calculator">10 * 2</gadget>
<output>20</output>

<gadget id="calculator">40 - 20</gadget>
<output>20</output>

<gadget id="calculator">1_000 / 5 / 20</gadget>
<output>10</output>

<gadget id="calculator">3 * 10</gadget>
<output>30</output>

<gadget id="calculator">30 + 20</gadget>
<output>50</output>

<gadget id="calculator">40 * 50</gadget>
<output>2_000</output>

<result>2_000</result>