1. **State the problem:** We are given that the price of a plain bowl of phở is $b$ and the price of each extra ingredient is $n$. The total price of a plain bowl plus 3 extra ingredients is 13. 2. **Write the equation:** The total cost can be expressed as: $$b + 3n = 13$$ 3. **Find possible values:** To find possible values of $b$ and $n$, we can express $b$ in terms of $n$: $$b = 13 - 3n$$ 4. **Interpretation:** Since prices cannot be negative, both $b$ and $n$ must be greater than or equal to 0: $$b \geq 0, \quad n \geq 0$$ 5. **Range of $n$:** From $b \geq 0$: $$13 - 3n \geq 0 \implies 3n \leq 13 \implies n \leq \frac{13}{3} \approx 4.33$$ 6. **Summary:** Possible values of $b$ and $n$ satisfy: $$b = 13 - 3n, \quad 0 \leq n \leq 4.33, \quad b \geq 0$$ This means for any $n$ in that range, $b$ is determined by the equation above.