ANSWER
The median is 19.5
Step-by-step explanation
In simple terms, the median is the middle number of a data set when the set is ordered. For a set with an odd amount of data, this is easy as it would the middle number. However, we have a set with an even amount of data.
The data entries 18 and 21 are surrounding the middle of the data set. Therefore, the median is the average of 18 and 21; that is, we add them together and divide by 2:
median = (18+21)/2 = 19.5
Therefore, the median of this data set is 19.5