**Quote:**

The median of List 1 is 11.

The median of List 1 is 10

List 1: 7, 8, 12, 15

**Median** – if the number of the elements in a set is odd, then it is the middle number of the set arranged in ascending or descending order;

if the number of the elements in the set is even, then it is the average of the two numbers in the middle of the arranged set.

List 1 is arranged in the ascending order and has four elements, which is an even number. Therefore the median is the average of the two middle numbers, 8 and 12.

(8 + 12) / 2 = 10

P.S. The median outside of the high school mathematics can be defined in a different way, but in GMAT we use only the definition provided above.