1. **Problem statement:** Find the difference between Neptune's and Mercury's distances from the sun. 2. **Given data:** - Distance of Mercury from the sun: $57,000,000$ km - Distance of Neptune from the sun: (not given explicitly, but implied to be larger) 3. **Formula:** To find the difference between two quantities, we use subtraction: $$\text{Difference} = \text{Distance of Neptune} - \text{Distance of Mercury}$$ 4. **Explanation:** Since the problem asks for the difference, the correct operation is subtraction (option b). 5. **Intermediate step:** If we denote Neptune's distance as $N$, then $$\text{Difference} = N - 57,000,000$$ 6. **Conclusion:** The difference between Neptune's and Mercury's distances from the sun is found by subtracting Mercury's distance from Neptune's distance. --- 1. **Problem statement:** Find the average debt per person in the US in the year 2000. 2. **Given data:** - Public debt: $5.6 \times 10^{12}$ dollars - Population: $2.8 \times 10^{8}$ people 3. **Formula:** Average debt per person is total debt divided by population: $$\text{Average debt per person} = \frac{\text{Total debt}}{\text{Population}}$$ 4. **Explanation:** The correct operation is division (option d). 5. **Calculation:** $$\frac{5.6 \times 10^{12}}{2.8 \times 10^{8}} = \frac{5.6}{2.8} \times \frac{10^{12}}{10^{8}} = 2 \times 10^{4} = 20,000$$ 6. **Conclusion:** The average debt per person was $20,000$ dollars. **Final answers:** - For question 20: Subtract (option b) - For question 21: Divide (option d)