I already posted an essay about the puzzles I wrote myself. Four of my five puzzles are math-related, so I am including them below for completeness. I will mention the topic of each puzzle unless it is a spoiler.

I start with Nikoli-type puzzles. Four elegant Nikoli-type puzzles were written or cowritten by Denis Auroux. In all of them the rules of the logic are stated at the beginning. That means the logic part doesn’t contain a mystery and can be solved directly.

- Good Fences Make Sad and Disgusted Neighbors (by Denis Auroux). You can guess by the title that this puzzle was in the emotions round corresponding to sadness and disgust. This is an interesting variation on the hexagonal Slitherlink. This is a relatively easy puzzle.
- Shoal Patrol (by Denis Auroux and James Douberley). Each grid is a combination of Battleship, Minesweeper, and a loop puzzle. These are difficult, but satisfying puzzles. The extraction step is not mathematical and not completely trivial.
- Submarine Patrol (by Denis Auroux and James Douberley). This is a 3D version of Shoal Patrol.
- Hashiwokakuro (Count your bridges) (by Denis Auroux). This is a mixture of Hashi and Kakuro. I enjoyed the puzzle while I tested it. The extraction is trivial.
- A Learning Path (by Tanya Khovanova and Xavid). This is a path logic puzzle that was targeted for new hunters. It contains self-referencing hints and solving techniques.

There were several puzzles that were very mathematical.

- X Marks the Spot (by Zachary Chroman). A very non-trivial puzzle about triangle centers.
- Tournament Organization (by Scott Harvey-Arnold ). A tournament reconstruction puzzle.
- Cash Cab (by Erica Newman and Justin Melvin).
- Voter Fraud (By Ben Weissmann). A voting puzzle.
- The 10,000 Puzzle Tesseract (by Charles Steinhardt, Zachary Chroman, and Scott Harvey-Arnold ). The most difficult puzzle of the hunt, by far.
- Games Club (by Tanya Khovanova and Sergei Bernstein). This puzzle is about combinatorial two-player games.
- Murder at the Asylum (by Tanya Khovanova). This is a difficult puzzle about liars and truth-tellers.
- Studies in Two-Factor Authentication (by Brandon Avila). A very elegant puzzle, that is one of my favorites.

There were also some math-related or computer-sciency puzzles.

- The Next Generation (by Colin Liotta). I enjoyed being an editor of this puzzle.
- Disorientation (by Alex Churchill). This puzzle has a beautiful visual component.
- Message in a Bottle (by Nathan Fung). The puzzle doesn’t look like it has something to do with mathematics, but my testing of it was very satisfying. I guessed from the start what it was about.
- Self-Referential Mania (by Justin Melvin). Self-referential logic puzzle, which I enjoyed editing.
- Bark Ode (by Elizabeth French, Justin Melvin, and Erica Newman). The pictures are so cute.
- Executive Relationship Commandments (by Robin Deits, John Toomey, and Michele Pratusevich). I didn’t see this puzzle until after the hunt. I wish I could have tested this puzzle with my son Alexey, who is a computer scientist.

There were also several decryption puzzles:

- Word Search (by Tanya Khovanova). A crypto word search.
- Texts From Mom (by Elizabeth French and Justin Melvin ). A text enciphered with emojis.
- Marked Deck (by Colin Liotta and Leland Aldridge). One of my favorite puzzles. Hunters received a physucal deck of cards that was laser cut. You can buy the deck at Etsy. The art in this puzzle is beautiful, but the puzzle also has a non-trivial decryption step.

Puzzle.Five pirates discovered a treasure of 100 gold coins. They decide to split the coins using the following scheme. The most senior pirate proposes how to share the coins, and all the pirates vote for or against it. If 50% or more of the pirates vote for it, then the coins will be shared that way. Otherwise, the pirate proposing the scheme will be thrown overboard, and the process is repeated with the next most senior pirate making a proposal.As pirates tend to be a bloodthirsty bunch, if a pirate would get the same number of coins whether he votes for or against a proposal, he will vote against so that the pirate who proposed the plan will be thrown overboard. Assuming that all five pirates are intelligent, rational, greedy, and do not wish to die, how will the coins be distributed?

You can find the solution in many places including Wikipedia’s Pirate game. The answer is surprising: the most senior pirate gets 98 coins, and the third and the fifth pirates by seniority get one coin each. I always hated this puzzle, but never bothered to think through and figure out why. Now I know.

This puzzle emphasizes the flaws of majority voting. The procedure is purely democratic, but it results in extreme inequality.

That means a democracy needs to have a mechanism to prohibit the president from blatantly benefiting himself. With our current president these mechanisms stopped working. Given that Trump does everything to enrich himself, the pirates puzzle tells us what to expect in the near future.

We, Americans, will lose everything: money, clean air and water, national parks, future climate, health, social security, and so on, while Trump will make money.

Share:]]>- A Learning Path (jointly with Xavid) is a Nikoli-type logic puzzle that was targeted for new hunters. It contained self-referencing hints and solving techniques. It was in both disgust and fear.
- Word Search is a word search with a twist. The words are related to fear and sadness.
- Games Club (jointly with Sergei Bernstein) is a puzzle on the topic of combinatorial two-player games. It is pure sadness.

I also wrote another easy puzzle called A Tribute: 2010-2017 (jointly with Justin Melvin, Wesley Graybill, and Robin Diets ). Though the puzzle is easy, it is useful in solving it to be familiar with the MIT mystery hunt. This is why the puzzle didn’t fit the first emotions round.

I also wrote a very difficult puzzle called Murder at the Asylum. This is a monstrosity about liars and truth-tellers.

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Problem.A number has three hundred ones and three hundred zeroes. Can it be a square?

The solution goes like this. Consider divisibility of this number by 9. The sum of the digits is 300. That means the number is divisible by 3, but not by 9. Therefore, it can’t be a square.

Why do we consider divisibility by 9? The divisibility by 9 is a very powerful tool, but why was it the first thing that came to my mind? The divisibility by 9 doesn’t depend on the order of the digits. Whenever I see a problem where the question talks about digits that can be in any order, the first tool to use is the divisibility by 9.

The why question, is very important in mathematics. But it is also very important in life. It took me many years to start asking why people did this or that. I remember my mom was visiting me in the US. Every time I came back from work, she complained that she was tired. Why? Because she did the laundry in the bath tub. She wouldn’t use my washing machine, because she didn’t have such a thing in Russia. I promised her that I’d do the laundry myself when there was a sufficient pile. However, she insisted that the dirty clothes annoyed her. I would point that my water bill went up. And so on.

We argued like this every day. We were both frustrated. Then I asked myself why. Why does she do the laundry? The answer was there. She wanted to be helpful. I calmed down and stopped arguing with her. I sucked it up and paid the water bills. Her time with me turned into the most harmonious visit we ever had. Unfortunately, it was the last.

Share:]]>**Puzzle.** Alice, Bob, and Charlie are at Alice’s house. They are going to Bob’s house which is 33 miles away. They have a 2-seat scooter which rides at 25 miles per hour with 1 rider on it; or, at 20 miles per hour with 2 riders. Each of the 3 friends walks at 5 miles per hour. How fast can all three of them make it to Bob’s house?

I can count to 1023 on my 10 fingers. The rudest number is 132.

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I kept forgetting my password, so I changed it to “incorrect”. Now, when I make a mistake during login, my computer reminds me: “Your password is incorrect.”

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—You promised me 8% interest, and in reality it is 2%.

—2 is 8—to some degree.

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Quantum entanglement is simple: when you have a pair of socks and you put one of them on your left foot, the other one becomes the “right sock,” no matter where it is located in the universe.

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Teacher:

—I keep telling my students that one half can’t be larger or smaller than the other. Still the larger half of my class doesn’t get it.

Puzzle.The professor is watching across a field how the son of the professor’s father is fighting with the father of the professor’s son. How is this possible?

This puzzle is tricky only because of gender-bias. Most people assume that the professor is male and miss the obvious intended solution, in which a female professor is watching her brother fighting with her husband.

I just gave this problem on a test. Here are other answers that I received.

- The professor is gay and is watching his brother fighting with his husband.
- The professor is watching his brother fighting with the father of the professor’s step-son.
- The father of the professor’s son is himself. So he is watching a video of himself fighting with his brother.

Years ago people couldn’t figure out this puzzle at all. So there has been progress. I was glad that my students suggested so many ideas that work. Nonetheless, many of them revealed their gender-bias by initially assuming that the professor is a man.

I can’t wait until this puzzle stops being tricky.

Share:]]>**Puzzle.** There are five houses of different colors next to each other equally spaced on the same road. In each house lives a man of a different profession.

- The blue house is adjacent to the mathematician’s and con-man’s houses.
- The first house on the left is green.
- The nurse lives immediately to the right of the mathematician.
- The teacher lives halfway between the plumber’s house and the yellow house.
- The nurse’s house is immediately to the right of the red house.

Who lives in the white house?

*Correction Nov 11, 2017. Replaced “the same distance from” with “halfway between” to eliminate the possibility of the plumber living in the yellow house. Thank you to my readers for catching this mistake and to Smylers for suggesting a correction.*

Problem.Invent a connected shape made out of squares on the square grid that cannot be cut into dominoes (rectangles with sides 1 and 2), but if you add a domino to the shape then you can cut the new bigger shape.

This problem reminds me of another famous and beautiful domino-covering problem.

Problem.Two opposite corner squares are cut out from the 8 by 8 square board. Can you cover the remaining shape with dominoes?

The solution to the second problem is to color the shape as a chess board and check that the number of black and white squares is not the same.

What is interesting about the first problem is that it passes the color test. It made me wonder: Is there a way to characterize the shapes on a square grid that pass the color test, but still can’t be covered in dominoes?

Share:]]>Don’t anthropomorphize computers: They don’t like it.

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I do not have dreams any more. What did I do wrong to make them delete my account?

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How to restore justice: Create a folder named Justice. Delete it. Go to the trash bin and click restore.

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An asocial network: When you sign up, you are friends with everyone. Then you send un-friend requests.

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