What is the perimeter of a rectangle with a length of 15 decimeters and a width of 6 decimeters?
一个长方形，长15分米，宽6分米，它的周长是多少．
<gadget id="calculator">15 + 6</gadget>
<output>21</output>

<gadget id="calculator">21 * 2</gadget>
<output>42</output>

<result>42</result>