1. **State the problem:** We need to find the mean of the data set: 20, 21, 22, 24 students per class. 2. **Formula for mean:** The mean (average) of a set of numbers is given by $$\text{Mean} = \frac{\text{Sum of all data points}}{\text{Number of data points}}$$ 3. **Calculate the sum:** $$20 + 21 + 22 + 24 = 87$$ 4. **Count the data points:** There are 4 data points. 5. **Calculate the mean:** $$\text{Mean} = \frac{87}{4}$$ 6. **Simplify the fraction:** $$\text{Mean} = \frac{\cancel{87}}{\cancel{4}} = 21.75$$ 7. **Final answer:** The mean number of students per class is **21.75**. **Note:** The second question about the median is not solved as per instructions to solve only the first question.