If the perimeter of a rectangle is 40 cm and its width is 7 cm, what is its length?
一个长方形的周长是40厘米，它的宽是7厘米，它的长是多少．
<gadget id="calculator">40 / 2</gadget>
<output>20</output>

<gadget id="calculator">20 - 7</gadget>
<output>13</output>

<result>13</result>