the first step is to order the numbers in ascending order:
15, 33, 39, 48, 67, 80, 90
to find quartiles, split the data into two. the first quartile is the median of the bottom half. (second quartile is median of the whole unit, and third q is median of the top half)
i.e. the median of 15, 33, 39
which is 33 :)
