The students collected tree species. There were 4 classes in the third grade, and each class collected 20 kilograms; there were 3 classes in the fourth grade, and each class collected 28 kilograms. How many kilograms were collected less in the third grade than in the fourth grade?
同学们采集树种，三年级有4个班，每班采集了20千克；四年级有3个班，每班采集了28千克，三年级比四年级少采集多少千克？
<gadget id="calculator">28 * 3</gadget>
<output>84</output>

<gadget id="calculator">20 * 4</gadget>
<output>80</output>

<gadget id="calculator">84 - 80</gadget>
<output>4</output>

<result>4</result>