A rectangle has a perimeter of 30 cm and its length is twice its width. Find the area of this rectangle.
一个长方形的周长是30厘米，长是宽的2倍．求这个长方形的面积．
<gadget id="calculator">1 + 2</gadget>
<output>3</output>

<gadget id="calculator">30 / 2 / 3</gadget>
<output>5</output>

<gadget id="calculator">5 * 2 * 5 * 1</gadget>
<output>50</output>

<result>50</result>