Concept:The mark-up is based on cost, while the margin is based on sales. Convert the mark-up into a margin first.Explanation:Mark-up =31 means profit is 1 part for every 3 parts of cost.So, cost =3 parts and profit =1 part.Therefore, sales =cost+profit=3+1=4 parts.Profit margin on sales =salesprofit=41.Given sales =Le180,000.Profit =41×Le180,000=Le45,000.The stock figures given are extra information and are not needed for this calculation.Answer:Gross profit =Le45,000.Correct option: B. Le 45,000.