Arbor Day is here, and the fourth grade task is to plant 300 trees. Class 4 and Class 1 acted first and planted 120 trees, and the remaining tasks were completed by Class 2 and Class 3. How many trees can each of these two classes plant on average to complete the remaining tasks?
植树节来了，四年级的植树任务是300棵．四1班先行动，已经植树120棵，剩下的任务由二班和三班完成．这两个班平均每班植树多少棵才能完成余下的任务？
<gadget id="calculator">300 - 120</gadget>
<output>180</output>

<gadget id="calculator">180 / 2</gadget>
<output>90</output>

<result>90</result>