The students sold waste paper. Class 51 sold 28 kilograms, and Class 2 sold 31 kilograms. The income of Class 51 was 2.4 yuan less than that of Class 52. How much does it cost per kilogram of waste paper?
同学们卖废纸，五1班卖了28千克，五2班卖了31千克，五1班的收入比五2班少2.4元．每千克废纸卖多少元？
<gadget id="calculator">31 - 28</gadget>
<output>3</output>

<gadget id="calculator">2.4 / 3</gadget>
<output>0.8</output>

<result>0.8</result>