The average of 5 numbers is 7.2, the average of the first 3 numbers is 7, the average of the last 3 numbers is 9, what is the middle number?
5个数的平均数是7.2，前3个数的平均数是7，后3个数的平均数是9，中间一个数是多少？
<gadget id="calculator">7 * 3</gadget>
<output>21</output>

<gadget id="calculator">9 * 3</gadget>
<output>27</output>

<gadget id="calculator">21 + 27</gadget>
<output>48</output>

<gadget id="calculator">7.2 * 5</gadget>
<output>36</output>

<gadget id="calculator">48 - 36</gadget>
<output>12</output>

<result>12</result>