The average age of Xiaomin, Xiaodong and Beibei is 9 years old, and the average age of Xiaomin and Beibei is 10 years old. So how old is Xiaodong?
小敏、晓冬、贝贝三人的平均年龄是9岁，小敏、贝贝两人的平均年龄是10岁．那么晓冬的年龄是多少岁．
<gadget id="calculator">9 * 3</gadget>
<output>27</output>

<gadget id="calculator">10 * 2</gadget>
<output>20</output>

<gadget id="calculator">27 - 20</gadget>
<output>7</output>

<result>7</result>