For a square vegetable field, if one group of opposite sides is increased by 15 meters during expansion, its area will be increased by 600 square meters. What is the original area of this vegetable field in square meters?
一块正方形菜地，扩建时把一组对边增加15米，它的面积就增加600平方米．这块菜地原来的面积是多少平方米？
<gadget id="calculator">600 / 15</gadget>
<output>40</output>

<gadget id="calculator">40 * 40</gadget>
<output>1_600</output>

<result>1_600</result>