Website maintenance costs, website design costs, and staff training costs do not change with the sales or turnover of an e-commerce company. So we can consider these costs as fixed.
Net sales=$(15000-800)=$14200 less: Gross profit margin@40% on sales=$5680 Cost of goods sold=$(14200-5680)=$8520 less: Purchase of goods b/w 1st jan to 4th jan=$7900 Inventory value as on 31st dec=$(8520-7900)=$620