Find the median of the following set of numbers: 5, 11, 11, 13, 17, 21.

The given numbers are $5, 11, 11, 13, 17, 21$. They are already in ascending order.

There are $n=6$ numbers in the list.

Since the number of values ($n=6$) is an even number, the median is the average of the two middle values. The positions of these values are $\frac{n}{2}$ and $\frac{n}{2} + 1$.
The positions are $\frac{6}{2} = 3$ and $\frac{6}{2} + 1 = 4$.
So, the median is the average of the 3rd and 4th numbers in the ordered list.

The ordered list is: $5, 11, \underline{11}, \underline{13}, 17, 21$.
The 3rd number is 11.
The 4th number is 13.

The final answer is 12.