Math Riddle: Two wine merchants arrive at the gates of Paris. One has 64 and the other 20 barrels of wine. Since they have not enough money to pay the customs duties, the first pay 40 francs and 5 barrels of wine. The second pays 2 barrels of wine but receives 40 francs in change.
What is the value of each barrel of wine and what is the duty payable?
Answer: The correct answer is the value of a barrel is 120 francs and the duty is 10 francs a barrel.