If the width of the rectangle is 4 cm and the length is twice the width, what is the perimeter in cm?
长方形的宽是4厘米，长是宽的2倍，周长是多少厘米．
<gadget id="calculator">4 * 2</gadget>
<output>8</output>

<gadget id="calculator">4 + 8</gadget>
<output>12</output>

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

<result>24</result>