1. **Problem Statement:** In a city, 20 percent of the population travels by car, 50 percent travels by bus, and 10 percent travels by both car and bus. We need to find the percentage of persons traveling by car or bus. 2. **Formula Used:** To find the percentage of people traveling by car or bus, we use the formula for the union of two sets: $$\text{Car or Bus} = \text{Car} + \text{Bus} - \text{Both Car and Bus}$$ 3. **Explanation:** This formula accounts for the overlap (people counted twice) by subtracting the intersection once. 4. **Substitute the values:** $$= 20 + 50 - 10$$ 5. **Calculate:** $$= 70$$ 6. **Interpretation:** 70 percent of the population travels by car or bus. **Final answer:** 70 percent (Option D)