The observations 6, 8, $x$, 15, 18 are written in ascending order and their median is 11. What is the mean of the data?

**Step 1**: Arrange data in ascending order.
**Step 2**: Count items (n).
**Step $3a$**: If $n$ is ODD, median is the value at position $(n+1)/2$.
**Step $3b$**: If $n$ is EVEN, median is the average of values at positions $n/2$ and $(n/2+1)$.

**Example odd $n\ast \ast :3,5,7,9,11.n=5$. Position $(5+1)/2=3$. Median = 7.

**Example even $n\ast \ast :4,6,8,10.n=4$. Positions 2 and 3 → values 6 and 8 → median = $(6+8)/2=7$.

Median is the right choice when there are extreme outliers, since it's not pulled by them like the mean is.