There are seven numbers arranged in descending order. The average of the first four numbers is 7.3, the average of the last four numbers is 4.5, and the average of these seven numbers is 6. What is the fourth number?
有七个数按从大到小排列，前四个数的平均数是7.3，后四个数的平均数是4.5，这七个数的平均数是6，那么第4个数多少．
<gadget id="calculator">7.3 * 4</gadget>
<output>29.2</output>

<gadget id="calculator">4.5 * 4</gadget>
<output>18</output>

<gadget id="calculator">6 * 7</gadget>
<output>42</output>

<gadget id="calculator">18 - 42</gadget>
<output>-24</output>

<gadget id="calculator">29.2 + (-24)</gadget>
<output>5.2</output>

<result>5.2</result>