Consider a sample with data values of 10, 20, 21, 17, 16, and 12. Compute...

The mean is average value of all the samples, in other words, it is equal to sum of the sample values devided by number of the values included.
${\displaystyle \bar{x}=\frac{10+20+21+17+16+12}{6}=\frac{96}{6}=16}$
To compute median, first sort the values:
${\displaystyle 10,12,16,17,20,21}$
There is even number of values (6), so the median is equal to the average of the two middle values in sorted data:
${\displaystyle Median=\frac{16+17}{2}=\frac{33}{2}=16.5}$