Riddle of the Week

Riddle of the Week: Le Havre to New York

Difficulty level: Moderate

Michael Stillwell
 
By Jay Bennett

This week we look at a riddle that was dreamed up by 18th-century French mathematician Édouard Lucas. According to a 1915 French math book, the story goes like this: Lucas was attending a science conference, and after lunch, several noted mathematicians were standing around. Lucas spoke up and challenged them to the following puzzle. A few guessed wrong, most remained silent, and no one answered correctly.

PROBLEM

Every day at noon an ocean liner leaves Le Havre to sail for New York, and (simultaneously) an ocean liner sails from New York to Le Havre. The crossing takes exactly seven days and seven nights in either direction. How many other ocean liners will an ocean liner leaving Le Havre today pass at sea before arriving in New York?

HINT

If you're wondering whether a ship casting off from harbor just as another arrives counts as a "pass at sea," yes, the solution does count these encounters.