How many square decimeters of iron sheet should be used to make a rectangular ventilation pipe with a length of 4m, a width and a height of 2cdm?
做一节长4m，宽和高都是2cdm的长方形通气管，至少要用多少平方分米的铁皮？
<gadget id="calculator">4 * 10</gadget>
<output>40</output>

<gadget id="calculator">2 * 40 * 4</gadget>
<output>320</output>

<result>320</result>