Buy exactly 100 animals for exactly 100 coins, and only one mix works
A horse costs 3 coins, a cow costs 1 coin, and sheep cost 1 coin for every 24 you buy. Spend exactly 100 coins to buy exactly 100 animals in total, using at least one of each kind. How many horses, cows and sheep?
Reveal the answer
23 horses, 29 cows, and 48 sheep: 23 + 29 + 48 = 100 animals, and 23×3 + 29×1 + 48÷24 = 69 + 29 + 2 = 100 coins. It's Problem 38 of Alcuin's 9th-century Propositiones ad Acuendos Juvenes, the oldest known collection of mathematical puzzles written in Latin.
— Alcuin of York, Propositiones ad Acuendos Juvenes — c. 800 CE, Problem 38