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Explanation:
The five number summary is the set of these items, in this exact order
So with the five number summary 4.6, 14.3, 19.7, 26.1, 31.2, we see that
Q1 = 14.3 and Q3 = 26.1
Subtracting these two values gets us the IQR (interquartile range)
IQR = Q3 - Q1
IQR = 26.1 - 14.3
IQR = 11.8