1. **State the problem:** We need to 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 the mean:** The mean is calculated by summing all the values and then dividing by the number of values. $$\text{Mean} = \frac{\text{Sum of all scores}}{\text{Number of scores}}$$ 3. **Calculate the sum:** $$26 + 25 + 31 + 23 + 22 + 19 + 21 + 27 + 18 + 25 + 27 + 28 + 24 + 28 + 23 + 21 + 29 + 28 + 28 + 21 + 18 = 530$$ 4. **Count the number of scores:** There are 21 scores in total. 5. **Calculate the mean:** $$\text{Mean} = \frac{530}{21} \approx 25.24$$ 6. **Interpretation:** The average pre-test score is approximately 25.24.