The average age of A, B, and C is 17 years old, and the average age of A and B is 15 years old, so what is the age of C?
甲、乙、丙三人的平均年龄为17岁，而甲乙两人的平均年龄为15岁，那么丙的年龄是多少岁．
<gadget id="calculator">3 * 17</gadget>
<output>51</output>

<gadget id="calculator">2 * 15</gadget>
<output>30</output>

<gadget id="calculator">51 - 30</gadget>
<output>21</output>

<result>21</result>