If the length of a rectangle is 30 cm and the width is 10 cm, what is its perimeter in cm?
一个长方形的长是30厘米，宽是10厘米，它的周长是多少厘米．
<gadget id="calculator">30 + 10</gadget>
<output>40</output>

<gadget id="calculator">40 * 2</gadget>
<output>80</output>

<result>80</result>