1. **State the problem:** Calculate the mean (average) of the given data set A: 102, 105, 122, 125, 127, 129, 130, 144, 155. 2. **Formula for mean:** The mean of a data set is given by $$\text{Mean} = \frac{\text{Sum of all data points}}{\text{Number of data points}}$$ 3. **Calculate the sum of the data points:** $$102 + 105 + 122 + 125 + 127 + 129 + 130 + 144 + 155 = 1139$$ 4. **Count the number of data points:** There are 9 data points. 5. **Calculate the mean:** $$\text{Mean} = \frac{1139}{9}$$ 6. **Simplify the fraction:** $$\text{Mean} = \frac{\cancel{1139}}{\cancel{9}} = 126.555\ldots$$ 7. **Round off to two decimal places:** $$\text{Mean} \approx 126.56$$ **Final answer:** The mean of data set A is **126.56**.