To find the interquartile range you would first have to find the median. If you list the number in least to greatest order you will find that the median is 30.5. After you find the median you will spilt the data set up. So on one side it will be 20, 25, 30 and 30, the other side you be 31, 40, 41, and 49. Next, you would find the median of the first set that I listed which is 27.5. You would do the same for the second set and find the median which is 40.5. You would then subtract the first sets median from the second sets median which is 40.5-27.5 and that is 13.
