There are 4 numbers. The average of the first two numbers is 7, the average of the middle two numbers is 2.3, and the last two numbers are 8.4. What is the average of the first and last numbers?
有4个数前两个数的平均数是7，中间两个数平均数是2.3，最后两个数是8.4，问，第一个数和最后一个数的平均数是多少？
<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>