In the "Arbor Day" tree planting activity, the first group has 15 people, and each person planted 5 trees on average. The second group of 20 people planted 140 trees in total. The third group has 15 people and planted 85 trees in total. How many trees are planted per person on average?
"植树节"植树活动中，第一小组15人，平均每人植树5棵．第二小组20人，共植树140棵．第三小组15人，共植树85棵．平均每人植树多少棵？
<gadget id="calculator">15 * 5</gadget>
<output>75</output>

<gadget id="calculator">75 + 140 + 85</gadget>
<output>300</output>

<gadget id="calculator">15 + 20 + 15</gadget>
<output>50</output>

<gadget id="calculator">300 / 50</gadget>
<output>6</output>

<result>6</result>