The perimeter of a rectangle is 30 meters, the width is 6 meters, and the length is in meters.
一个长方形的周长是30米，宽是6米，长是多少米．
<gadget id="calculator">30 / 2</gadget>
<output>15</output>

<gadget id="calculator">15 - 6</gadget>
<output>9</output>

<result>9</result>