The average age of A, B, and C is 15 years old, and the average age of A and B is 13 years old. How old is C?
甲、乙、丙三人的平均年龄是15岁，甲、乙两人的平均年龄是13岁，丙是多少岁．
<gadget id="calculator">3 * 15</gadget>
<output>45</output>

<gadget id="calculator">2 * 13</gadget>
<output>26</output>

<gadget id="calculator">45 - 26</gadget>
<output>19</output>

<result>19</result>