Final answer:
The median of the set of numbers is 20.
Step-by-step explanation:
The median of the set of numbers {10, 12, 12, 12, 13, 18, 20, 25, 26, 27, 30, 30, 31} is 20.
To find the median, first arrange the numbers in ascending order: 10, 12, 12, 12, 13, 18, 20, 25, 26, 27, 30, 30, 31.
Since the set has 13 numbers, the median is the middle number, which in this case is the 7th number, 20.