The average of the three numbers A, B, and C is 4.5, and the sum of the two numbers A and B is 8.4. What is the number of C?
甲、乙、丙三个数的平均数为4.5，甲、乙两数之和是8.4，丙数是多少．
<gadget id="calculator">4.5 * 3</gadget>
<output>13.5</output>

<gadget id="calculator">13.5 - 8.4</gadget>
<output>5.1</output>

<result>5.1</result>