The school organized students in the third, fourth and fifth grades to plant trees on Arbor Day, and a total of 840 trees were planted. The third, fourth, and fifth grades each have 4 classes. How many trees are planted in each class on average?
学校在植树节组织三、四、五年级的同学去植树，共植树840棵．三、四、五年级各有4个班，平均每个班植树多少棵？
<gadget id="calculator">3 * 4</gadget>
<output>12</output>

<gadget id="calculator">840 / 12</gadget>
<output>70</output>

<result>70</result>