The three classes of the fourth grade are going to plant trees. A total of 300 trees will be planted. 180 trees have been planted. The rest will be done in three times. How many trees are planted on average each time?
四年级三个班去植树，一共要栽300棵树，已栽了180棵，剩下的分三次完成，平均每次栽多少棵树？
<gadget id="calculator">300 - 180</gadget>
<output>120</output>

<gadget id="calculator">120 / 3</gadget>
<output>40</output>

<result>40</result>