The length of a rectangle is 12 cm, the width is 8 cm, what is the circumference in cm?
一个长方形的长是12厘米，宽是8厘米，周长是多少厘米．
<gadget id="calculator">12 + 8</gadget>
<output>20</output>

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

<result>40</result>