Write four numbers in a row. The average of the first two numbers is 7, the average of the middle two numbers is 2.3, and the average of the last two numbers is 8.4. What is the average of the first and last numbers?
把四个数写成一行，前两个数的平均数是7，中间两个的平均数是2.3，最后两个数的平均数是8.4，第一个和最后一个的平均数是多少．
<gadget id="calculator">7 * 2</gadget>
<output>14</output>

<gadget id="calculator">8.4 * 2</gadget>
<output>16.8</output>

<gadget id="calculator">2.3 * 2</gadget>
<output>4.6</output>

<gadget id="calculator">16.8 - 4.6</gadget>
<output>12.2</output>

<gadget id="calculator">14 + 12.2</gadget>
<output>26.2</output>

<gadget id="calculator">26.2 / 2</gadget>
<output>13.1</output>

<result>13.1</result>