A set of data 23, 27, 20, 18, X, 12, their median is 21, then what is X?
一组数据23、27、20、18、X、12，它们的中位数是21，那么X是多少．
<gadget id="calculator">21 * 2</gadget>
<output>42</output>

<gadget id="calculator">42 - 20</gadget>
<output>22</output>

<result>22</result>