The median is the middle number, after the numbers have been put in order.
If there are two numbers in the middle then the median is the number half-way
between them. This can be found by adding them together and dividing by 2.

4, 5, 7, 12

The two numbers in the middle are 5 and 7.
The number half-way between 5 and 7 is 6, so the median is 6.