What is the perimeter of a rectangle whose length is 8 cm and width is 4 cm?
长为8厘米，宽为4厘米的长方形，它的周长是多少．
<gadget id="calculator">8 + 4</gadget>
<output>12</output>

<gadget id="calculator">12 * 2</gadget>
<output>24</output>

<result>24</result>