Median

The median is the middle of a sorted data set — half the values fall below it and half above.
The very first step is always to put the values in order.

When the count is odd, the median is the single middle value.
You can find its position with $\frac{n + 1}{2}$: for $13$ values, that is the $7$th value once sorted.

When the count is even, there are two middle values, and the median is their average.
For the sorted set $1, 4, 6, 9$, the two middle values are $4$ and $6$, so the median is $\frac{4 + 6}{2} = 5$.

You can also read the median off a dot plot or frequency table without listing every value.
First add up all the dots to get the total, then count in from the left until you reach the middle position.
In the plot below there are $11$ dots, so the median is the $6$th one; counting from the left, the $6$th dot sits over $2$, so the median is $2$.

What is the median of the set $3, 8, 12, 20$?
There are four values (an even count), so the median is the average of the two middle ones, $8$ and $12$.
So the median is $\frac{8 + 12}{2} = 10$.

A sorted data set has $13$ values. Which value is the median?
With an odd count, the median is at position $\frac{n + 1}{2} = \frac{13 + 1}{2} = 7$.
So the $7$th value is the median.

What is the median of the set $20, 3, 8, 12, 7$?
First sort the values: $3, 7, 8, 12, 20$.
With five values, the median is the single middle one, so the median is $8$.