Please continue to write a number so that the average of the four numbers is 6; how many are 5, 7, and 9.
请接着写出一个数，使四个数的平均数是6；5，7，9多少．
<gadget id="calculator">6 * 4</gadget>
<output>24</output>

<gadget id="calculator">5 + 7 + 9</gadget>
<output>21</output>

<gadget id="calculator">24 - 21</gadget>
<output>3</output>

<result>3</result>