Average stock is derived by adding

The average stock is derived by adding the opening stock to the closing stock and then dividing the sum by two. The opening stock is the value of inventory at the beginning of an accounting period, while the closing stock is the value of inventory at the end of the accounting period. The average stock is the value of inventory that a business holds on average during the accounting period. By adding the opening and closing stock and then dividing by two, a business can calculate the average stock value. This calculation is used in various accounting and financial ratios, such as inventory turnover ratio and gross profit ratio. These ratios help a business to analyze its inventory management and profitability. The other options provided in the question are not correct methods to calculate the average stock value. The first option is the average of closing stock and purchases, which does not include the opening stock. The second option is the ratio of closing stock to opening stock, which does not provide an average value. The fourth option is the difference between opening stock and purchases, which also does not include the closing stock value.