The perimeter of a rectangular desk is 40 decimeters, the width is 8 decimeters, and how many decimeters is the length?
长方形的课桌的周长是40分米，宽是8分米，长是几分米？
<gadget id="calculator">40 / 2</gadget>
<output>20</output>

<gadget id="calculator">20 - 8</gadget>
<output>12</output>

<result>12</result>