1. **State the problem:** Find the mode of the data set: 25, 22, 24, 24, 24, 21, 28, 21, 25, 24, 21, 22, 21, 25, 22, 24, 25, 25, 21, 25, 25. 2. **Recall the definition:** The mode is the value that appears most frequently in a data set. 3. **Count the frequency of each number:** - 21 appears 5 times. - 22 appears 3 times. - 24 appears 5 times. - 25 appears 7 times. - 28 appears 1 time. 4. **Identify the number with the highest frequency:** 25 appears 7 times, which is more than any other number. 5. **Conclusion:** The mode of the data set is $25$ because it occurs most frequently.