To make a cuboid iron box without a cover, its length is 3.5 decimeters, its width and height are both 1 decimeter, at least how many square decimeters of iron sheet should be used.
做一个无盖的长方体铁盒，它的长是3.5分米，宽和高都是1分米，至少要用多少平方分米的铁皮．
<gadget id="calculator">3.5 * 1</gadget>
<output>3.5</output>

<gadget id="calculator">3.5 * 1 * 2</gadget>
<output>7</output>

<gadget id="calculator">1 * 1 * 2</gadget>
<output>2</output>

<gadget id="calculator">3.5 + 7 + 2</gadget>
<output>12.5</output>

<result>12.5</result>