The interquartile range, IQR, is the difference between the third interquartile, Q3, and the first interquartile, Q1.
This is: IQR = Q3 - Q1.
The first interquartile is the 25% percentile and the third interquartile is the 75% percentile.
Order the data: 20, 34, 56, 67, 78, 91, 124.
In those data the median is 67
The first interquartile, Q1 is the median of the values from 20 to 67, which is (34 + 56) / 2 = 45.
The third interquartile, Q3, is the median from 67 to 124 = (78 + 91) / 2 = 84.5
IQR = 84.5 - 45 = 39.5
Answer: 39.5