1. **State the problem:** We need to find the unknown amount of tuna $A$ (in kilograms) using a proportion with the given values $A$, 36, 8, and 9. 2. **Set up the proportion:** The problem suggests a proportion of the form $$\frac{A}{8} = \frac{9}{36}$$ where $A$ corresponds to the unknown amount of tuna, 8 and 36 are quantities related to the tuna, and 9 is the cost associated with the unknown amount. 3. **Explain the proportion:** A proportion states that two ratios are equal. Here, the ratio of the unknown amount $A$ to 8 is equal to the ratio of 9 to 36. 4. **Solve the proportion:** Cross-multiply to solve for $A$: $$A \times 36 = 9 \times 8$$ $$36A = 72$$ 5. **Isolate $A$:** Divide both sides by 36: $$\frac{\cancel{36}A}{\cancel{36}} = \frac{72}{36}$$ $$A = 2$$ 6. **Interpret the result:** The amount of tuna that costs 9 is $2$ kilograms.