What are the minimum first quartile median third quartile and the maximum of the data set? 9,20,4,18,4,18,20,9

Answers
1. Minimum = 4
2. First quartile = 6.5
3. Median = 13.5
4. Third quartile = 19
5. Maximum = 20

Explanation
To calculate the measure of central tendency, you first arrange the set of the data in ascending order.  
The set of data given will be;
4, 4, 9, 9, 18, 18, 20, 20.

Part 1:
The minimum value of the data is 4.

Part 2:
The first quatile is the median of the lower half which is comprised by:
4, 4, 9, 9

1st quartile = (4+9)÷2
                  = 13÷2
                  = 6.5

Part 3:
Median of the data is;

Median = (9+18)÷2
              =27÷2
            = 13.5

Part 4:
3rd quartile is the median of the upper half which comprises of; 
18, 18, 20, 20.

3rd quartile = (18+20)÷2
                  = 48÷2
                   = 19

Part 5
The maximum of the set of data is 20.

Were is the problem