Concept:Mark-up is gross profit expressed as a percentage of the cost of sales.Explanation:First, calculate the cost of sales.Cost of sales = Opening stock + Purchases − Closing stockCost of sales = 60,000+230,000−40,000=250,000Next, calculate the gross profit.Gross profit = Sales − Cost of salesGross profit = 450,000−250,000=200,000Now, apply the mark-up formula.Mark-up = Cost of salesGross profit×100Mark-up = 250,000200,000×100=80%Answer:B. 80%