1. **State the problem:** Find the mean (average) of the given pre-test scores: 26, 25, 31, 23, 22, 19, 21, 27, 18, 25, 27, 28, 24, 28, 23, 21, 29, 28, 28, 21, 18. 2. **Formula for mean:** The mean $\bar{x}$ is calculated by summing all the values and dividing by the number of values: $$\bar{x} = \frac{\sum x}{n}$$ where $\sum x$ is the sum of all scores and $n$ is the number of scores. 3. **Calculate the sum:** $$26 + 25 + 31 + 23 + 22 + 19 + 21 + 27 + 18 + 25 + 27 + 28 + 24 + 28 + 23 + 21 + 29 + 28 + 28 + 21 + 18 = 532$$ 4. **Count the number of scores:** There are $n = 21$ scores. 5. **Calculate the mean:** $$\bar{x} = \frac{532}{21} \approx 25.3333$$ 6. **Interpretation:** The average pre-test score is approximately 25.33.