The average of the eight numbers is 2.1, the average of the first three numbers is 2.6, and the average of the last four numbers is 1.4. What is the other number?
八个数的平均数是2.1，前三个数的平均数为2.6，后四个数的平均数为1.4，还有一个数是多少？
<gadget id="calculator">2.1 * 8</gadget>
<output>16.8</output>

<gadget id="calculator">2.6 * 3</gadget>
<output>7.8</output>

<gadget id="calculator">1.4 * 4</gadget>
<output>5.6</output>

<gadget id="calculator">16.8 - 7.8 - 5.6</gadget>
<output>3.4</output>

<result>3.4</result>