An ordered data set is 4, 7, 9, 12, 15, 18, 21, 24, 30. What is its inter-quartile range?

1. The inter-quartile range is the third quartile minus the first quartile. 2. There are nine values, and they are already in ascending order. 3. The first quartile lies one quarter of the way into the list, at position nine plus one divided by four, which is 2.5. 4. Position 2.5 sits between the second value, 7, and the third value, 9, so the first quartile is 8. 5. The third quartile lies three quarters of the way in, at position three multiplied by ten divided by four, which is 7.5. 6. Position 7.5 sits between the seventh value, 21, and the eighth value, 24, so the third quartile is 22.5. 7. The inter-quartile range is 22.5 minus 8, which is 14.5. 8. The answer 26 is the trap for giving the full range of 30 minus 4, the answer 15 quotes the median instead, and the answer 12 reads off the values at positions 3 and 7 without averaging the neighbouring pairs. _Source: Siyavula Mathematics Grade 12 (Everything Maths, CC BY 4.0), Chapter 9: Statistics_