The average of three numbers is 8.4, the first number is 9.6, and the second number is 1.2 times the first 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，第二个数是第一个数的1.2倍，那么第三个数是多少．
<gadget id="calculator">8.4 * 3</gadget>
<output>25.2</output>

<gadget id="calculator">9.6 * 1.2</gadget>
<output>11.52</output>

<gadget id="calculator">25.2 - 9.6 - 11.52</gadget>
<output>4.08</output>

<result>4.08</result>