Find the median of the set of numbers below.​​      15, 16, 15, 7, 21, 18, 19, 20, 1115, 16, 15, 7, 21, 18, 19, 20, 11

To find the median of a set of numbers, follow these steps:

1. First, arrange the numbers in ascending order: 7, 7, 11, 11, 15, 15, 15, 15, 16, 16, 18, 18, 19, 19, 20, 20, 21, 21.

2. Then, find the middle number. If there is an odd number of observations, the median is the middle number. If there is an even number of observations, the median is the average of the two middle numbers.

In this case, there are 18 numbers, so we need to find the average of the 9th and 10th numbers.

3. The 9th number is 15 and the 10th number is 16.

4. To find the average of these two numbers, add them together and divide by 2: (15 + 16) / 2 = 15.5.

So, the median of this set of numbers is 15.5.