A rectangular test field has a circumference of 30 meters and a length of 9 meters. What is its area in square meters?
一块长方形试验田，周长30米，长9米，它的面积是多少平方米？
<gadget id="calculator">30 / 2</gadget>
<output>15</output>

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

<gadget id="calculator">9 * 6</gadget>
<output>54</output>

<result>54</result>