Something Fishy at Customs

One of two men had 12 fish, and the other had 13 fish, and all of the fish were of the same price. From the first man, a custom agent took away one fish and 12 denari for payment. And from the other he took two fish and gave him back 7 denari. Find the customs fee and the price of each fish.

Liber Abaci, Leonardo of Pisa, 1202

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