The length of a rectangle is 20 mm, the width is 10 mm, what is the perimeter in mm?
长方形的长是20毫米，宽是10毫米，周长是多少毫米．
<gadget id="calculator">20 + 10</gadget>
<output>30</output>

<gadget id="calculator">30 * 2</gadget>
<output>60</output>

<result>60</result>