Am i not supposed to add the cost of sales and(opening inventory - closing inventory) given

Here's why:

• The "cost of sales" figure that is given is already the Cost of Goods Sold (COGS). It represents the final calculated cost of the goods that were sold during the period.
• The formula involving opening inventory, purchases, and closing inventory (i.e., $\text{Opening Inventory} + \text{Purchases} - \text{Closing Inventory}$) is used to calculate the Cost of Goods Sold if the COGS is not directly provided.

You would use one method or the other, not both:

• Method 1 (if COGS is given directly as "cost of sales"): Use the given "cost of sales" figure as your COGS. The inventory figures are then redundant for the gross margin calculation.
• Method 2 (if COGS is NOT given directly, but purchases and inventory figures are): Calculate COGS using $\text{Opening Inventory} + \text{Purchases} - \text{Closing Inventory}$.

You do not combine the two. If "cost of sales" is provided, it's the final COGS value you need for the gross margin calculation.