A man, his wife, and two children must cross a river in a boat built for one adult
A man and his wife, exactly equal in weight, want to cross a river with their two children, each of whom weighs exactly half as much as a parent. They have only one small boat, and it can carry no more than the weight of a single adult at a time. How do all four of them get across?
Reveal the answer
It takes 9 crossings. The two children cross first; one rows back. The father crosses alone; the other child rows back to fetch its sibling. Both children cross together again; one rows back once more. The mother crosses alone; and finally the remaining child rows the boat back one last time to bring the last child over. This is Proposition 19 of Alcuin of York's Propositiones ad Acuendos Juvenes ('Problems to Sharpen the Young'), an 8th-century Latin manuscript containing the earliest known river-crossing puzzles in Western mathematics — including two other famous ones involving jealous couples and a wolf, goat and cabbage.
— Alcuin of York, Propositiones ad Acuendos Juvenes — c. 800 CE, Proposition 19