The school plans to plant 999 trees on Arbor Day. Students in grades four, five, and six have already planted 699 trees. The rest will be completed by six classes in grade four. How many trees are planted in each class on average?
学校植树节时准备栽999棵树，四、五、六年级的同学们已经栽了699棵，剩下的由四年级的6个班完成，平均每个班栽几棵树？
<gadget id="calculator">999 - 699</gadget>
<output>300</output>

<gadget id="calculator">300 / 6</gadget>
<output>50</output>

<result>50</result>