The median is the middle number in the data set when
the data set is written from least to greatest.
So let's write our data set from least to greatest.
4, 9, 15, 16, 18, 21, 25, 27, 30, 31, 33
Now, we identify the middle number.
4, 9, 15, 16, 18, 21, 25, 27, 30, 31, 33
Notice that 21 appears in the middle.
So the median of the data set is 21.