The average of three numbers is 8.4. The first number is 9.6, which is twice the second number. What is the third number?
三个数的平均数是8.4，其中第一个数是9.6，是第二个数的2倍，第三个数是多少．
<gadget id="calculator">8.4 * 3</gadget>
<output>25.2</output>

<gadget id="calculator">9.6 / 2</gadget>
<output>4.8</output>

<gadget id="calculator">25.2 - 9.6 - 4.8</gadget>
<output>10.8</output>

<result>10.8</result>