If the length of a rectangle is 20 cm and the width is 15 cm, what is its perimeter in cm?
长方形的长是20厘米，宽是15厘米，它的周长是多少厘米．
<gadget id="calculator">20 + 15</gadget>
<output>35</output>

<gadget id="calculator">35 * 2</gadget>
<output>70</output>

<result>70</result>