Teachers and students in the fourth and fifth grades of Qishan Primary School accepted the task of planting trees on the mountain. 45 students from the fourth grade went there, and 15 students from the fifth grade went. A total of 900 trees were planted in the two grades. How many trees were planted per person on average?
奇山小学四、五年级师生接受上山植树任务，四年级去了45人，五年级去了15人，两级共植树900棵，平均每人植树多少棵？
<gadget id="calculator">45 + 15</gadget>
<output>60</output>

<gadget id="calculator">900 / 60</gadget>
<output>15</output>

<result>15</result>