Four travelers, one torch, one narrow bridge
Four travelers must cross a rickety bridge at night, sharing a single torch that must accompany every crossing; at most two people may cross at once, and a pair moves at the slower person's pace. Solo crossing times are 5, 10, 20, and 25 minutes. Can all four reach the other side in under an hour, and what's the fastest possible time?
Reveal the answer
60 minutes is optimal: the two fastest (5, 10) cross together (10 min); the 5 returns with the torch (5 min); the two slowest (20, 25) cross together (25 min); the 10 returns (10 min); then the 5 and 10 cross together again (10 min): 10+5+25+10+10=60. The puzzle's earliest known print appearance is Saul X. Levmore and Elizabeth Early Cook's 1981 book; it later became a well-worn tech-industry interview riddle.
— Saul X. Levmore and Elizabeth Early Cook, Super Strategies for Puzzles and Games — Doubleday, 1981 — earliest documented print appearance of the puzzle
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