There is a set of data 23, 27, 30, 18, M, 12, if their median is 21, then what is the data M
有一组数据23、27、30、18、M、12中，若它们的中位数是21，那么数据M是多少
<gadget id="calculator">21 * 2</gadget>
<output>42</output>

<gadget id="calculator">42 - 23</gadget>
<output>19</output>

<result>19</result>