If the length of a rectangle is 12 meters and its width is 4 meters, what is its perimeter?
一个长方形的长是12米，比宽长4米，它的周长是多少米？
<gadget id="calculator">12 - 4</gadget>
<output>8</output>

<gadget id="calculator">12 + 8</gadget>
<output>20</output>

<gadget id="calculator">20 * 2</gadget>
<output>40</output>

<result>40</result>