If the length of a rectangle is 7 cm and the width is 4 cm less than the length, what is its perimeter?
一个长方形的长是7厘米，宽比长少4厘米，它的周长是多少．
<gadget id="calculator">7 - 4</gadget>
<output>3</output>

<gadget id="calculator">7 + 3</gadget>
<output>10</output>

<gadget id="calculator">10 * 2</gadget>
<output>20</output>

<result>20</result>