Final answer:
The interquartile range (IQR) of the given data set is 46.
Step-by-step explanation:
The interquartile range (IQR) is a measure of the spread of data within the middle 50% of a dataset. It is calculated as the difference between the third quartile (Q3) and the first quartile (Q1). In this data set (2, 5, 9, 11, 18, 30, 42, 48, 55, 73, 81), the first quartile is 9 and the third quartile is 55. Therefore, the IQR is 55 - 9 = 46.