Calculate the gross profit using the gross profit mark-up percentage:Gross Profit = Sales Amount * (Gross Profit Mark-up Percentage / 100)Gross Profit = D12,000 * (25 / 100)Gross Profit = D12,000 * 0.25Gross Profit = D3,000Calculate the cost of sales by subtracting the gross profit from the sales amount:Cost of Sales = Sales Amount - Gross ProfitCost of Sales = D12,000 - D3,000Cost of Sales = D9,000Therefore, the cost of sales is D9,000.