The length of a rectangle is 90 meters and the width is 60 meters. Dig a square pool with a side length of 30 meters in the middle of it, and plant grass and greenery around it. How many square meters is the green area?
一块长方形地的长是90米，宽是60米．在他的中间挖一个边长是30米的正方形水池，周围种草绿化．绿化部分的面积是多少平方米？
<gadget id="calculator">90 * 60</gadget>
<output>5_400</output>

<gadget id="calculator">30 * 30</gadget>
<output>900</output>

<gadget id="calculator">5_400 - 900</gadget>
<output>4_500</output>

<result>4_500</result>