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

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

<result>22</result>