A merchant's 100-coin shopping list for 100 animals has exactly one solution
A merchant travelling east wants to buy exactly 100 animals for exactly 100 coins. A camel costs 5 coins, a donkey costs 1 coin, and sheep cost 1 coin for every 20 bought. Using at least one of each, how many camels, donkeys and sheep did he buy?
Reveal the answer
19 camels, 1 donkey, and 80 sheep: 19 + 1 + 80 = 100 animals, and 19×5 + 1×1 + 80÷20 = 95 + 1 + 4 = 100 coins. It's Problem 39 of Alcuin's 9th-century puzzle collection, immediately following its near-twin about horses, cows and sheep.
— Alcuin of York, Propositiones ad Acuendos Juvenes — c. 800 CE, Problem 39