A rectangular vegetable field with a circumference of 18 meters has a ratio of length to width of 5:4. What is the area of the vegetable field?
一个周长是18米的长方形菜地，长和宽的比是5：4．求菜地的面积是多少？
<gadget id="calculator">18 / 2</gadget>
<output>9</output>

<gadget id="calculator">5 + 4</gadget>
<output>9</output>

<gadget id="calculator">5 / 9</gadget>
<output>5/9 = around 0.555556</output>

<gadget id="calculator">9 * (5/9)</gadget>
<output>5</output>

<gadget id="calculator">4 / 9</gadget>
<output>4/9 = around 0.444444</output>

<gadget id="calculator">9 * (5/9) * 9 * (4/9)</gadget>
<output>20</output>

<result>20</result>