There is a rectangle. If the length is increased by 0.5 meters and the width remains the same, the area will increase by 2.5 square meters; if the width is increased by 0.8 meters and the length remains the same, the area will be increased by 6.4 square meters. What is the original area of the rectangle?
有一个长方形，如果长增加0.5米，宽不变，则面积增加2.5平方米；如果宽增加0.8米，长不变，则面积增加6.4平方米．长方形原来的面积是多少平方米？
<gadget id="calculator">6.4 / 0.8</gadget>
<output>8</output>

<gadget id="calculator">2.5 / 0.5</gadget>
<output>5</output>

<gadget id="calculator">8 * 5</gadget>
<output>40</output>

<result>40</result>