1. The problem is to calculate the interquartile range (IQR) given the five number summary: minimum = 18, lower quartile (Q1) = 20, median = 21, upper quartile (Q3) = 23, maximum = 24. 2. The formula for the interquartile range is: $$\text{IQR} = Q3 - Q1$$ 3. Substitute the given values into the formula: $$\text{IQR} = 23 - 20$$ 4. Calculate the difference: $$\text{IQR} = 3$$ 5. The interquartile range measures the spread of the middle 50% of the data, which helps understand the variability without being affected by extreme values. Final answer: The interquartile range is 3.