If you take an air-conditioned bus in a certain city, you need to insert a coin of 3 yuan per person. If you only pay 2.4 yuan, what percentage is the discount for swiping a card compared to using a coin?
乘坐某城市的空调公交车每人需投币3元，如果刷卡只扣费2.4元，刷卡比投币优惠百分之几？
<gadget id="calculator">3 - 2.4</gadget>
<output>0.6</output>

<gadget id="calculator">0.6 / 3</gadget>
<output>0.2</output>

<result>0.2</result>