What is the perimeter of a rectangle whose width is 3 cm and whose length is twice its width?
一个长方形的宽是3厘米，长是宽的2倍，它的周长是多少．
<gadget id="calculator">3 * 2</gadget>
<output>6</output>

<gadget id="calculator">6 + 3</gadget>
<output>9</output>

<gadget id="calculator">9 * 2</gadget>
<output>18</output>

<result>18</result>