When a firm operates at an output level of 9,000 units, the…

When a firm operates at an output level of 9,000 units, the per-unit cost is $5. When the production is between 10,000–12,000 units, the per-unit cost is $4. At a production level of 13,000 units, the production cost is again $5 per unit. At 14,000 units and above, the production cost increases further. At what output level does the firm experience economies of scale?