If the length of a rectangle is 8 cm and the width is 4 cm, what is its perimeter in cm?
一个长方形的长是8厘米，宽是4厘米，它的周长是多少厘米．
<gadget id="calculator">8 + 4</gadget>
<output>12</output>

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

<result>24</result>