1. **Problem 4:** Given the equation $$520 \div x = 40n$$, find an expression equal to $$nx$$. 2. **Step 1:** Start with the given equation: $$520 \div x = 40n$$ which can be rewritten as $$\frac{520}{x} = 40n$$. 3. **Step 2:** Multiply both sides by $$x$$ to isolate 520: $$\cancel{x} \times \frac{520}{\cancel{x}} = 40n \times x$$ which simplifies to $$520 = 40nx$$. 4. **Step 3:** Divide both sides by 40 to solve for $$nx$$: $$\frac{520}{40} = \frac{40nx}{40}$$ $$\cancel{40} \times \frac{520}{\cancel{40}} = \cancel{40} nx \div \cancel{40}$$ which simplifies to $$13 = nx$$. 5. **Answer:** The expression equal to $$nx$$ is $$13$$. **Final answer: A. 13**