Recreational Mathematics

[Man] is only completely a man when he plays.
Friedrich von Schiller   (1759-1805) 

• Counterfeit Coin: In 3 weighings, find an odd object among 12, 13 or 14.
  • Unabridged discussion.
• Counterfeit Penny Problem:  Find an odd object in the fewest weighings.
  • Unabridged discussion.
• Seven-Eleven: Four prices with a sum and product both equal to 7.11.
• Equating a right angle and an obtuse angle, with a clever false proof.
• Choosing a raise: Trust common sense, beware of  fallacious accounting.
• 3 men pay $30 for a $25 hotel room, the bellhop keeps $2...  Is $1 missing?
• Chameleons: A situation is unreachable because of an invariant quantity.
• Sam Loyd's 14-15 puzzle also involves an invariant quantity (and 2 orbits).
• Einstein's riddle: 5 distinct colors, nationalities, drinks, smokes and pets.
• Numbering n pages of a book takes this many digits (formula).
• The Ferry Boat Problem (by Sam Loyd): To be or not to be ingenious?
• Hat overboard !   What's the speed of the river?
• All digits once and only once: 48 possible sums (or 22 products).
• 2-people bridge crossed by 4 people (U2).  Four paces, one flashlight!
• Managing supplies to travel 6 days while carrying enough for only 4 days.
• Go south, east, north and you're back... not necessarily to the North Pole!
• Icosapolis: Numbering a 5 by 4 grid so adjacent numbers differ by at least 4.
• Unusual mathematical boast for people born in 1806, 1892, or 1980.
• Puzzles for extra credit: From Chinese remainders to the Bookworm Classic.
• Simple geometrical dissection:  A proof of the Pythagorean theorem.
• Early bird saves time by walking to meet incoming chauffeur.
• Sharing a meal: A man has 2 loaves, the other has 3, a stranger has 5 coins.
• Fork in the road: Find the way to Heaven by asking only one question.

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• Proverbial Numbers: Words commonly associated with some numbers.
• Riddles: The Riddle of the Sphinx and other classics, old and new.

Related articles on this site:

• Martin Gardner's puzzles.
• Dice.
• Mathematical "Magic" Tricks.
• Deceits and Illusions.
• How many Bachet squares?
  A 1624 puzzle, using the 16 court cards.
• Mathematical Games: Winning Strategies.
• The Game of Chess.
• Ternary Guessing Cards.
• Decimal Arcana:  The world of  decadic integers.
• Humor:  Math Jokes, Funny Units, Funny Prefixes...
• Carmichael Multiples of odd numbers coprime to their totients, up to 9997.
• The puzzle of Thales:  How to find the height of the  Great Pyramid ?
• Utility functions: A rational basis for decisions under uncertain conditions.
• Assigning a definite value to a  divergent series.

Related Links (Outside this Site) Mathpuzzle.com, by Ed Pegg Jr: Penrose's Unilluminable Room (2007-07-29) Jaap's Puzzle Page, by  Jaap Scherphuis. Braingle brain teasers.  |  Ludimaths (French) George Peter Jelliss  |  Mike Keith Recreational Mathematics by David Eppstein (UC Irvine)  |  PuzzleDepot Hooda Math  Educational games for grades  1-8. Brainteasers, Puzzles & Quizzes, for Nurses at www.nursefriendly.com. The Well-Posed Puzzle   |   Jim Loy's Puzzle Page   |   Grey Labyrinth BrainBashers   |   Math Magic, by  Erich Friedman. Project Euler, by  Colin Hughes  (MathsChallenge.net).   Eugène Strens Recreational Mathematics Collection (University of Calgary) Math Puzzles in Sam Loyd's Cyclopedia listed by Donald Knuth (UC Stanford). Dudeney's Puzzles in The Weekly Dispatch (1896-1904), listed by  Don Knuth. The Canterbury Puzzles  by Henry E. Dudeney  (Project Gutenberg  eBook[an error occurred while processing this directive]