Students in the third and fourth grades participated in tree planting activities. Three classes in the third grade planted 108 trees in total, and four classes in the fourth grade planted a total of 168 trees. How many fewer trees were planted in each class in the third grade than in the fourth grade?
三、四年级学生参加植树活动，三年级3个班共植树108棵，四年级4个班共种168棵，三年级比四年级平均每班少种多少棵？
<gadget id="calculator">168 / 4</gadget>
<output>42</output>

<gadget id="calculator">108 / 3</gadget>
<output>36</output>

<gadget id="calculator">42 - 36</gadget>
<output>6</output>

<result>6</result>