The students collect tree species. There were 6 classes in the third grade, and each class collected 20 kilograms; there were 5 classes in the fourth grade, and each class collected 28 kilograms. How many kilograms did the third grade collect less than the fourth grade?
同学们采集树种．三年级有6个班，每班采集了20千克；四年级有5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 * 5</gadget>
<output>140</output>

<gadget id="calculator">20 * 6</gadget>
<output>120</output>

<gadget id="calculator">140 - 120</gadget>
<output>20</output>

<result>20</result>