A rectangular piece of land is 12 meters long and 9 meters wide. A fence is built around it. How long is the fence?
一块长方形地、长12米，宽9米．四周围上篱笆，篱笆长多少米？
<gadget id="calculator">12 + 9</gadget>
<output>21</output>

<gadget id="calculator">21 * 2</gadget>
<output>42</output>

<result>42</result>