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WELCOME to [wu:riddles]! This is an archive of problems I've been collecting since the Spring of 2002. They come from many places, including word of mouth, college courses, books, and job interviews for hi-tech positions. Many are even written by members of our own forum community. These carefully chosen puzzles will demand you to think in creative ways you perhaps normally would not. In fact, some will seem outright impossible at first ... but once you crack them, the epiphany can be truly rapturous!

If you are a first-timer, here are some problems I recommend you start out with:

The riddles are organized by difficulty as judged by myself: easy, medium, and hard. Then there is the microsoft section, consisting of weird, open-ended consulting-style questions. The cs and putnam sections contain problems which may require a particularly specialized background in computer science and mathematics, respectively. Later, I will add sections for "What Am I" and "What Happened" riddles.

Hints are available for some riddles. To see a hint, highlight the area near the word "Hint:" with your mouse. This mouse-highlighting convention is used in both the forums and this cover site.

If you need solutions or more hints, please post to the FORUM! Almost all puzzles are solved in great detail there. Also, if you want to contribute a new puzzle, please post to the FORUM as well. We have a daily healthy discussion of new problems, and every once in a while I will export some of those problems to the cover site. The forum is a little slow, and I apologize for this, but please be patient. Alternatively, you can send me an e-mail directly. (Remove the "NO_SPAM_" prefix.) However, my response time may be very slow, as I'm often busy these days; the forum is definitely preferred.

My favorite kind of puzzle satisfies the following criteria: 1) It seems impossible, 2) Most people can understand it, and 3) It does not require specialized knowledge to solve. When I started collecting, I tried hard to find problems that satisfied these things. However, as the site has grown over the years, it has become apparent that for many of the harder problems, you won't have a chance without knowing some post-high-school mathematics. Basic probability and combinatorics are particularly common. If you lack this background, please do not be discouraged. Our friendly and active forum community can help you get up to speed. My sincere hope is that you will leave with a greater appreciation of the power of math.

Many thanks to all the cool people who have sent me new material. Also, I express much gratitude to towr and Icarus for their extensive work in generously maintaining the forums. I also thank David Lau, Eric Yeh, and James Fingas for contributing an exceptional number of problems when the site was getting started. Let's make this the definitive riddle archive.

Finally, I would like to again plug the forum, which i invite you to browse and perhaps even join. You will be very hard-pressed to find another problem-solving community of comparable quality. It is the neatest thing on this site, a giant board of discussions for thousands of puzzles. A LaTeXish syntax is also provided to support writing of mathematical notation. New problems arise almost every hour, and we have members all over the globe, at all ages, from professions ranging from graphic designers to engineering professors. Having the privilege of building such a community is one of the nicest things I've ever taken part of.

Whatever your reasons for being here -- whether you are a life-long mathemagician, or a desperate kid googling for tricky homework solutions -- I hope you will stick with this site regularly, and come to appreciate the joys of problem solving.

- William Wu, 4/2/2002 11:51PM; Revisions: 9/29/2002 7:24PM; 11:40 PM 8/19/2004, 12:55 PM 2/5/2005


MNeeds math past arithmetic and basic probability.
CRequires knowing how to play chess.
PPhysics knowledge is helpful.
>=PI don't know the solution to this problem myself.
CPURequires calculator/computer power.


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Check out latest puzzles by perusing the forum and the 10 most recent posts. Latest additions to cover site can be seen by clicking here.

Thanks to Contributors David Lau, Siddhartha Doshi, Jiong Shen, Carl Wang, Phoebus Chen, Alan Liu, Hansen Bow, Ernest Zhu, Elaine Lo, Yosen Lin, Don Barkauskas, Katherine Chan, Jasvir Nagra, Tau Beta Pi (TBP), Eta Kappa Nu (HKN), Danny Dulai, His Grace The Duke Of Ankh-Morpork Commander Sir Samuel Vimes, Michael Snyder, Dipan K. Ghosh, Eric Cole, Louis Wainwright, Ben Bardill, Patrick Dreker, Autumn Looijen, Stephen Champailler, Christopher Kings-Lynne, Bart Samwel, Kannan Ramchandran, Nick Yee, Steve Plimpton, Levsen Hendrik, Remco Hartog, [I.M._Smarter_Enyu], Philip Mock, Michael Chang, Jon Meilstrup, Ryan Russell, Matt Young, Jonathan Haas, Geoff Canyon, Peter Surda, Cory Ondrejka, Satish Rao, [gcooper], Ted Powell, Brave Sir Robin, Eric Cole, J. A. H. Hunter, Sean R. Owen, Andrew Glenn, Bruce Preston, Peter Ratiu, Michael Mendelsohn, Rob Mahurin, James Fingas, Bryan Organ, Jeroen Rutten, Stephen Montgomery-Smith, Marko Lukat, Eric Yeh, Nick Hobson, Mike Lawther, [anshil], Richard Feynman, Douglas Hofstaeder, Dacher Keltner, David Mace, [SAS], Matthew Schultheis, John Leen, Andrew Ooi, Folkert Hindriks, Steve Ragle, Daniel Filner, Karl Barrus, Misha Kruk, Keith Lloyd, Dave Minott, Jette Randlov, Eric Winger, Nathan Hellweg, Tom VanCourt, Chris Seaton, Mitchell Morris, Michael Styer, Zameer Andani, Jonathan Blow, Jeff Thompson, Jonathon Duerig, Dan Hanson, Gabriel Sechan, Tom Saxton, [HunterWare], [alsee], James Antill, Tom Barringer, Bart Massey, David Krikorian, Eric Sharkey, [tudorb], Kevin Day, Milan Ramaiya, Robert Merkel, James Jones, Haim Bitner, Adam Barth, Oscar Lazzarino, Damien Fisher, [DrkShadow], Erik Blankendaal, Eric Smith, James Demmel, Jonathan Shewchuk, Alex Harris, Michael Kelley, [Mr._Martingale], Kaisen Lin, Hakan Yilmaz, Freddy Mercury, Justin Rising, Marko Lukat, William Kahan, Jeremy Randolph, Michael Sinatra, David Messerschmitt, Patrick Masterson, Frederik Bonte, Randy Williams, Pietro K.C., Brett Danaher, Derek Abbott, Ralph Boleslavsky, Rui del-Negro, [college math journal], [amer. math monthly], Spyros Potamianos, Gary Hsieh, [rec.puzzles], Steven Rudich, Matt Lahut, Richard Reti, Paul Sinclair, Tim Mann, [ucb engineering news], Luke Percival, Anwis Das, Mike White, Louise Poon, Jeffrey Wilhelm, Anthony Cammon, [BNC], A.Frieze & D.Sleator, [SWF], Ted Stevens, Frank Wang, Danny P, Patrick Sesulka, [towr], Chi Sum Cheung, Ranjit Jhala, Jacob Scott, David McKay, Eamon Warnock (THUDandBLUNDER), Kozo Morimoto, Abhijit Joshi, Devesh Parekh, Amnon Melzer, Mary Lou, Leonid Brouhkis, Allistair Sinclair, Mark Newheiser, Joc Koelman, Paul Jung, Aryabhatta, Thomas Cover, Barukh, Nootch, Eigenray

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