The perimeter of a rectangle is 20 cm, its width is 4 cm, what is its length?
长方形的周长是20厘米，它的宽是4厘米，它的长是多少．
<gadget id="calculator">20 / 2</gadget>
<output>10</output>

<gadget id="calculator">10 - 4</gadget>
<output>6</output>

<result>6</result>