1. **State the problem:** Find the mode of the data set: 11, 12, 12, 9, 9, 11, 9, 15, 15, 10, 11, 12, 11, 12, 12, 13, 11, 12, 13, 12, 12, 11, 9. 2. **Recall the definition of mode:** The mode is the value that appears most frequently in a data set. 3. **Count the frequency of each number:** - 9 appears 4 times - 10 appears 1 time - 11 appears 6 times - 12 appears 8 times - 13 appears 2 times - 15 appears 2 times 4. **Identify the number with the highest frequency:** The number 12 appears 8 times, which is more than any other number. 5. **Conclusion:** The mode of the data set is $12$ because it occurs most frequently.