1. **State the problem:** A dirt bike requires 15 liters of gas mixed with 4 liters of oil. We need to find how much oil is needed if 20 liters of gas are used. 2. **Set up the ratio:** The ratio of gas to oil is $15:4$. 3. **Write the proportion:** Let $x$ be the amount of oil needed for 20 liters of gas. Then: $$\frac{15}{4} = \frac{20}{x}$$ 4. **Cross multiply:** $$15 \times x = 4 \times 20$$ $$15x = 80$$ 5. **Solve for $x$:** $$x = \frac{80}{15}$$ 6. **Simplify the fraction:** $$x = \frac{\cancel{80}}{\cancel{15}} = \frac{16}{3} \approx 5.33$$ 7. **Interpret the result:** You will need approximately 5.33 liters of oil for 20 liters of gas. **Final answer:** $$\boxed{\frac{16}{3} \text{ liters of oil} \approx 5.33 \text{ liters}}$$