A sidewalk with a width of 4 meters covers an area of 640 square meters. After expansion, the width is doubled and the length remains the same. What is the area of the sidewalk after expansion?
一条4米宽的人行道，占地面积640平方米，扩建后，宽变为原来2倍，长不变，扩建后的人行道面积是多少平方米？
<gadget id="calculator">640 * 2</gadget>
<output>1_280</output>

<result>1_280</result>