What is the perimeter of a rectangle whose length is 20 decimeters and width 10 decimeters?
一个长方形，长是20分米，宽10分米，周长是多少．
<gadget id="calculator">20 + 10</gadget>
<output>30</output>

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

<result>60</result>