A rectangular vegetable field, the ratio of length to width is 7:3, and the width is 20 meters less than the length, what is the area of this vegetable field in square meters?
一块长方形菜地，长和宽之比是7：3，宽比长少20米，这块菜地的面积是多少平方米？
<gadget id="calculator">7 - 3</gadget>
<output>4</output>

<gadget id="calculator">20 / 4</gadget>
<output>5</output>

<gadget id="calculator">5 * 7</gadget>
<output>35</output>

<gadget id="calculator">5 * 3</gadget>
<output>15</output>

<gadget id="calculator">35 * 15</gadget>
<output>525</output>

<result>525</result>