The average of the two numbers A and B is 6.4, the average of the three numbers A, B, and C is 5.7, what is the number of C?
甲、乙两数平均数是6.4，甲、乙、丙三个数平均数是5.7，丙数是多少．
<gadget id="calculator">5.7 * 3</gadget>
<output>17.1</output>

<gadget id="calculator">6.4 * 2</gadget>
<output>12.8</output>

<gadget id="calculator">17.1 - 12.8</gadget>
<output>4.3</output>

<result>4.3</result>