The median is the central number of a data set; to find it we need to order the data set from smallest to largest, then we found the central number. For the last step we have two possible options:
If the data set has an even number of elements the median is the mean of the two central numbers.
If the data set has an odd number of elements the meadian is the central number.
For this case we have an even number of elements (10), hence the median will be the mean of the fifth and sixth elements of the ordered data set. The ordered data set is:
4, 5, 12, 18, 21, 28, 31, 35, 37, 48
From it we notice that the fifth and sixth elements are 21 and 28, respectively. Taking their mean we have:
Therefore, the median of the data set is 24.5