The Lucas Ship Problem
Every day at noon a ship leaves Le Havre for New York and another ship leaves New York for Le Havre. Each trip lasts seven days and seven nights. How many ships going east will a ship leaving Le Havre today meet during its journey to New York? Can you answer graphically?
This problem was invented by Edoard Lucas, a French nineteenth-century mathematician.
 B. A. Kordemsky, The Moscow Puzzles, London: Penguin, 1990 p. 108, 273.
"The Lucas Ship Problem"
Wolfram Demonstrations Project
Published: April 10 2014