There is a rectangular vegetable field in my grandfather's house. If the length is reduced by 5 meters, it will become a square, and the area will be reduced by 75 square meters. How many square meters is the original vegetable plot area?
爷爷家有一块长方形的菜地，如果把长减少5米就变成一个正方形，而且面积减少75平方米．原来菜地面积是多少平方米？
<gadget id="calculator">75 / 5</gadget>
<output>15</output>

<gadget id="calculator">15 + 5</gadget>
<output>20</output>

<gadget id="calculator">20 * 15</gadget>
<output>300</output>

<result>300</result>