The number of freshmen in the third and fourth grades of elementary school is equal. The average age of each person in the third grade is 9 years old, and the average age of each person in the fourth grade is 11 years old. What is the average age of each person in these two grades?
新生小学三、四年级人数相等，三年级平均每人9岁，四年级平均每人11岁，这两个年级平均每人多少岁．
<gadget id="calculator">9 + 11</gadget>
<output>20</output>

<gadget id="calculator">10 + 10</gadget>
<output>20</output>

<gadget id="calculator">20 / 20</gadget>
<output>1</output>

<gadget id="calculator">10 * 1</gadget>
<output>10</output>

<result>10</result>