The students participated in the tree planting labor. On the first day they planted 18 trees, on the second day they planted 10 trees in the morning, and in the afternoon they planted 12 trees. How many trees are planted per day on average?
同学们参加植树劳动，第一天植树18棵，第二天上午植树10棵，下午植树12棵．平均每天植树多少棵？
<gadget id="calculator">18 + 10 + 12</gadget>
<output>40</output>

<gadget id="calculator">40 / 2</gadget>
<output>20</output>

<result>20</result>