The price of green vegetables fell from 0.5 yuan per catty yesterday to 0.4 yuan per catty today. By what percentage has it dropped? what is the correct formula
青菜的价格从昨天的0.5元一斤跌到今天的0.4元一斤，跌了百分之几？正确的算式是多少
<gadget id="calculator">0.5 - 0.4</gadget>
<output>0.1</output>

<gadget id="calculator">0.1 / 0.5</gadget>
<output>0.2</output>

<result>0.2</result>