On Arbor Day, the fourth-grade students planned to plant 126 trees. They planted half of the total in 3 hours. How many trees are planted per hour on average?
植树节，四年级学生计划要种126棵树，他们用3小时种了总数的一半，平均每小时种多少棵？
<gadget id="calculator">3 * 2</gadget>
<output>6</output>

<gadget id="calculator">126 / 6</gadget>
<output>21</output>

<result>21</result>