If the perimeter of a rectangle is 40 decimeters and the length is 12 decimeters, what is its width?
一个长方形的周长是40分米，长是12分米，那么它的宽是多少分米．
<gadget id="calculator">40 / 2</gadget>
<output>20</output>

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

<result>8</result>