The average of A, B, and C is 17, the average of A and B is 15, what is the number of C?
甲、乙、丙三数的平均数是17，甲乙平均数是15，丙数是多少．
<gadget id="calculator">17 * 3</gadget>
<output>51</output>

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

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

<result>21</result>