$$\text{At EBQ, Total Setup Costs} = \text{Total Carrying Costs}$$
You can double-check this:
$$\text{Number of Batches} = \frac{4000}{300} = 13.33 \implies \text{Total Setup Cost} = 13.33 \times 900 = \text{Rs. } 12,000$$
$$\text{Average Inventory} = \frac{300}{2} = 150 \text{ units} \implies \text{Total Carrying Cost} = 150 \times 80 = \text{Rs. } 12,000$$
Because both costs are equal, the solution of 300 units is mathematically verified.