The school planted 300 trees of various kinds in the third and fourth grades, and 200 and 220 trees in the fifth and sixth grades respectively. On average, how many trees are planted in each grade.
学校植树，三，四年级各种树300棵，五、六年级分别种树200棵、220棵．平均每个年级种树多少棵．
<gadget id="calculator">300 * 2</gadget>
<output>600</output>

<gadget id="calculator">600 + 200 + 220</gadget>
<output>1_020</output>

<gadget id="calculator">1_020 / 4</gadget>
<output>255</output>

<result>255</result>