This year, the average age of A, B, and C is 13 years old. It is known that A is 14 years old, B is 13 years old, and how old is C.
今年甲、乙、丙三人的平均年龄是13岁．已知甲14岁，乙13岁，丙多少岁．
<gadget id="calculator">13 * 3</gadget>
<output>39</output>

<gadget id="calculator">39 - 14 - 13</gadget>
<output>12</output>

<result>12</result>