1. **State the problem:** We are given a set of numbers: 18, 21, 20, 12, 26, 19, 9. We need to find the range of this data set. 2. **Formula for range:** The range is the difference between the maximum and minimum values in a data set. $$\text{Range} = \text{Maximum value} - \text{Minimum value}$$ 3. **Identify the maximum and minimum values:** - Maximum value = 26 - Minimum value = 9 4. **Calculate the range:** $$\text{Range} = 26 - 9$$ 5. **Simplify the expression:** $$\text{Range} = \cancel{26} - \cancel{9} = 17$$ 6. **Final answer:** The range of the data set is **17**.