Customs

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 custom duties, the first pays 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 value of a barrel is 120 francs and the duty is 10 francs a barrel.

Source: https://www.riddles.com/3209

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 custom duties, the first pays 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? — Customs by don orr v1.