If the length of a rectangle is 20 cm and the width is 9 cm, what is its perimeter in cm?
一个长方形的长是20厘米，宽是9厘米，它的周长是多少厘米．
<gadget id="calculator">20 + 9</gadget>
<output>29</output>

<gadget id="calculator">29 * 2</gadget>
<output>58</output>

<result>58</result>