1. **State the problem:** We are given that at output $Q=3$, the average variable cost (AVC) is 10,000 and the average total cost (ATC) is 12,000. We need to find the average fixed cost (AFC) when output is $Q=4$. 2. **Recall the formula:** Average total cost is the sum of average variable cost and average fixed cost: $$\text{ATC} = \text{AVC} + \text{AFC}$$ This means $$\text{AFC} = \text{ATC} - \text{AVC}$$ 3. **Calculate AFC at $Q=3$:** Given $\text{AVC} = 10,000$ and $\text{ATC} = 12,000$ at $Q=3$, $$\text{AFC} = 12,000 - 10,000 = 2,000$$ 4. **Find total fixed cost (TFC):** Since $\text{AFC} = \frac{\text{TFC}}{Q}$, $$\text{TFC} = \text{AFC} \times Q = 2,000 \times 3 = 6,000$$ 5. **Calculate AFC at $Q=4$:** Total fixed cost does not change with output, so $$\text{AFC at } Q=4 = \frac{\text{TFC}}{4} = \frac{6,000}{4} = 1,500$$ **Final answer:** The average fixed cost when output is four units is $1,500$, which corresponds to option b.