1. **Stating the problem:** We need to find the 4th quartile (D4) of the given data set, which is the value at the 4th decile position. 2. **Understanding the formula:** The formula to find the position of the $k$-th decile in a data set of size $n$ is: $$\text{Position} = \frac{k}{10} \times (n + 1)$$ where $k=4$ for the 4th decile. 3. **Calculate the position:** The data has $n=10$ values. $$\text{Position} = \frac{4}{10} \times (10 + 1) = 0.4 \times 11 = 4.4$$ 4. **Sort the data values:** The values are: 70, 85, 90, 75, 80, 95, 85, 70, 80, 90 Sorted ascending: 70, 70, 75, 80, 80, 85, 85, 90, 90, 95 5. **Locate the 4.4th value:** This is between the 4th and 5th values in the sorted list. 4th value = 80 5th value = 80 6. **Interpolate to find D4:** $$D4 = 80 + 0.4 \times (80 - 80) = 80 + 0 = 80$$ 7. **Final answer:** The 4th decile (D4) of the data is **80**.