Take into account that the median of a data set is given by the element of the set that is at the center of the ordered list of elements. If there is no possible to determine a central element in the list, then, you take two elements of the center and calculate the average value in between such elements.
Then, first order the elements, as follow:
1 , 2 , 3 , 4 , 4 , 5 , 7 , 8 , 9 , 12
THe number of elements is 10, then, you conisder the two elements at the center of the list, that is, the 5th and 6th elements:
1 , 2 , 3 , 4 , 4 , 5 , 7 , 8 , 9 , 12
and calculate the average in between these numbers:
median = (4 + 5)2 = 9/2 = 4.5
Hence, the median of the given data set id 4.5