Four classes in the third grade of Experimental Primary School participated in the tree planting activity. On the first day, 18 trees were planted, on the second day 20 trees were planted, and on the third day 22 trees were planted. The following calculation formula is to calculate the average number of trees planted per class.
实验小学三年级4个班参加植树活动，第一天植树18棵，第2天植树20棵，第3天植树22棵．下面多少算式是求平均每班植树多少棵．
<gadget id="calculator">18 + 20 + 22</gadget>
<output>60</output>

<gadget id="calculator">60 / 4</gadget>
<output>15</output>

<result>15</result>