There are 4 classes in the fourth grade, and each class has 45 students. Students of all grades participated in tree planting activities, and an average of 5 trees were planted per person. How many trees did the fourth graders plant in total?
四年级有4个班，每班都是45人．全年级的学生参加植树活动，平均每人植树5棵．四年级学生共植树多少棵？
<gadget id="calculator">45 * 4</gadget>
<output>180</output>

<gadget id="calculator">5 * 180</gadget>
<output>900</output>

<result>900</result>