The average of 8 numbers is 2.1, the average of the first 3 numbers is 2.6, the average of the last 4 numbers is 1.4, what is the fourth number?
8个数的平均数是2.1，前3个数的平均数为2.6，后4个数的平均数为1.4，第四个数是多少？
<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>