The first quartile of this data will be 15, because 12 numbers are above it in the dataset, and 4 are below it, so 1/4 of the dataset is below it.
The third quartile will be 40, because 4 numbers are above it and 12 are below it (so 1/4 are above it).
The IQR (inter-quartile range) is the difference between these numbers, or 40-15=25.
