Posts tagged ‘puzzle’

Silent Letter Night

Several weeks ago, Will Shortz presented an NPR Sunday Puzzle in which he stated a word and a letter, and the resulting collection would be rearranged to form a new word in which the added letter is silent. For instance, if Will gave RODS + W, the correct answer would be SWORD, in which the W is silent. (Note that the collection of letters is also an anagram of WORDS, but the W isn’t silent.)

At a time of year known for silent nights, it seems like a puzzle involving silent letters is completely appropriate. I’ve borrowed Shortz’s idea and extended it a bit; some of the clues in the list below have more than one silent letter added. Many items in the list are related to today’s holiday; and, because this is a math blog, the others are related to mathematics. In full disclosure, two of the answers are proper nouns.

Enjoy, and happy holidays!

  1. TO + W =
  2. TON + K =
  3. TOGS + H =
  4. GEE + D =
  5. TIN + G + H =
  6. SIN + G =
  7. SIN + E =
  8. CORD + H =
  9. HOLE + W =
  10. HEART + W =
  11. TINNY + E =
  12. COINS + E =
  13. NOELS + M =
  14. PILES + E + L =
  15. FRAME + T =
  16. REDACT + E + S + S =
  17. RACISMS + H + T =

December 25, 2018 at 10:22 am Leave a comment

12 Math Games, Puzzles, and Problems for Your Holiday Car Trip

It doesn’t matter if you’re one of the 102.1 million people traveling by car, one of the 6.7 million people traveling by plane, or one of the 3.7 million people traveling by train, bus, or cruise ship this holiday season — the following collection of games, puzzles, and problems will help to pass the time, and you’ll be there long before anyone asks, “Are we there yet?”


1. Street Sign Bingo

Use the numbers on street signs to create expressions with specific values. For instance, let’s say you see the following sign:

Road Sign

The two numbers on the sign are 12 and 3, from which you could make the following:

12 + 3 = 15

12 – 3 = 9

12 × 3 = 36

12 ÷ 3 = 4

Of course, you could just use the numbers directly as 3 or 12. Or if your passengers know some advanced math, they could use square roots, exponents, and more to create other values.

Can you split the digits within a number and use them separately? Can you concatenate two single-digit numbers to make a double-digit number? That’s for you and your traveling companions to decide.

To play as a competitive game, have each person in the car try to get every value from 1‑20. It’s easiest to keep track if you require that players go in order. To make it a cooperative game, have everyone in the car work collectively to make every value from 1 to 100.

As an alternative, you could use the numbers on license plates instead.

2. Bizz Buzz Bang

Yes, I’ve played this as a drinking game. No, I don’t condone drinking while driving. No, I don’t condone under-age drinking, either. Yes, I condone playing this game with minors while driving. (See what I did there?)

The idea is simple. You pick two single-digit numbers, A and B. Then you and your friends start counting, one number per person. But each time someone gets to a number that contains the digit A or is a multiple of A, she says, “Bizz!” Every time someone gets to a number that contains the digit B or is a multiple of B, she says, “Buzz!” And every time someone gets to a number that meets both criteria, he says, “Bang!”

For example, let’s say A = 2 and B = 3. Then the counting would go like this:

one, bizz, buzz, bizz, five, bang, seven, bizz, buzz, bizz, eleven, bang, buzz, …

When someone makes a mistake, that round ends. Start again, and see if you can beat your record.

You can use whatever numbers you like, or modify the rules in other ways. For instance, what if bizz is for prime numbers and buzz is for numbers of the form 4n + 3? Could be fun!

3. Dollar Nim (or any other variation)

Nim is a math strategy game in which players take turns removing coins from a pile. Different versions of the game are created by adjusting the number of coins that can be removed on each turn, the number of coins in the pile originally, how many piles there are, and whether you win (normal) or lose (misère) by taking the last coin.

A good aspect of Nim is that you don’t actually need coins. To play in the car, players just need to keep track of a running total in their heads.

A version of Nim dubbed 21 Flags was played on Survivor Thailand several years ago. There were 21 flags, and each team could remove 1, 2, or 3 flags on each turn. The team to remove the last flag won. This is a good first version to play in the car, especially for young kids who have never played before. Then mix it up by changing the initial amount and the number that can be removed on each turn.

Our family’s favorite version, Dollar Nim, is played by starting with $1.00 and removing the value of a common coin (quarter, dime, nickel, penny) on each turn. (While discussing this post with my sons, they informed me that they much prefer Euro Nim, which begins with 1€, but then the coin values to be removed are 50c, 20c, 10c, 5c, 2c, and 1c. It’s essentially the same game, but they like the different coin amounts.)


Three-Pile Nim consists of not just one but three piles with 3, 5, and 7 coins, respectively.  On each turn, a player must remove at least one coin, and may remove any number of coins, but all removed coins must be from the same pile.

Finally, Doubling Nim is played as the name implies. On the first turn, a player may remove any number of coins but not the entire pile. On every turn thereafter, a player may remove any number of coins up to double the number taken on the previous turn. For instance, if your opponent removes 7 coins, then you can remove up to 14 coins.

For every version of Nim, there is an optimal strategy. We’ve wasted hours on car trips discussing the strategy for just one variation. Discussing the strategies for all the versions above could occupy the entire drive from Paducah to Flint.

4. Guess My Number

One person picks a number, others ask questions to try to guess the number.

The simplest version is using “greater than” and “less than” questions. Is it greater than 50? Is it less than 175? And so forth. Using this method, the guessers can reduce the number of possibilities by half with each question, so at most, it should take no more than n guesses if 2n > m, where m is the maximum possible number that the picker may choose. For instance, if the picker is required to choose a number less than 100, then n = 7, because 27 = 128 > 100. Truthfully, this version of the game gets boring quickly, but it’s worth playing once or twice, especially if there’s one picker and multiple guessers. And a conversation about the maximum number of guesses need can be a fun, mathy way to spend 15 minutes of your trip.

A more advanced version excludes “greater than” and “less than” questions. Instead, the guessers can ask other mathematical questions like, “Is it a prime number?” or “Do the digits of the number differ by 4?” Those questions imply, of course, that the answer must be yes or no, and that’s typical for these types of guessing games. If you remove that restriction, though, then guessers could ask questions that reveal a little more information, like, “What is the difference between the digits?” or “What is the remainder when the number is divided by 6?” With this variant, it’s often possible to identify the number with two strategic questions.


Here are three puzzles that can lead to hours of conversation — and frustration! — on a car trip. Before you offer one to your crew, though, put forth the disclaimer that anyone who’s heard the puzzle before must remain mum. No reason they should spoil the fun for the rest of you. (The puzzles are presented here without solution, because you’ll know when you get the right answer. You can find the answer to any of them online with a quick search… but don’t do that. You’ll feel much better if you solve it yourself.)

5. Dangerous Crossing

Rickety BridgeFour people come to a river in the middle of the night. There’s a narrow bridge, but it’s old and rickety and can only hold two people at a time. They have just one flashlight and, because it’s night and the bridge is in disrepair, the flashlight must be used when crossing the bridge. Aakash can cross the bridge in 1 minute, Britney in 2 minutes, Cedric in 5 minutes, and Deng in 8 minutes. When two people cross the bridge together, they must travel at the slower person’s pace. And they need to hurry, because zombies are approaching. (Oh, sorry, had I failed to mention the zombie apocalypse?) What is the least amount of time that all four people can cross the bridge?

And how can you be sure that your method is the fastest?

6. Weight of Weights

Marilyn has a simple balance scale and four small weights, each weighing a whole number of grams. With the balance scale and these weights, she is able to determine the weight of any object that weighs between 1 kg and 40 kg. How much does each of the four weights weigh?

7. Product Values

Assign each letter a value equal to its position in the alphabet, i.e., A = 1, B = 2, C = 3, …, Z = 26. Then for any common word, find its product value by multiplying the value of the letters in the word. For instance, the product value of CAT is 60, because 3 × 1 × 20 = 60.

  1. Find as many common English words as you can with a product value of 60.
  2. Find a common English word that has the same product value as your name. (A little tougher.)
  3. Find a common English word with a product value of 3,000,000. (Zoiks!)


8. The Three of Life

This one looks so innocent!

What’s the probability that a randomly chosen number will contain the digit 3?

But spend a little time with it. And prepare for. Mind. Blown.

9. Hip to Be (Almost) Square

No calculators for this one.

There are four positive numbers — 1, 3, 8, and x — such that the product of any two of them is one less than a square number. What is the least possible value of x?

No spreadsheets, either.

10. Hip to Be Square (Roots)

Just some good, old-fashioned algebra and logic to tackle this one.

Which of the following expressions has a greater value?

\sqrt{10} + \sqrt{11}  or  \sqrt{43}


11. Coming and Going

Potentially counterintuitive.

At the holidays, Leo drove to his grandma’s, and the traffic was awful! His average speed was 42 miles per hour. After the holidays, however, he drove home along the same route, and his average speed was 56 miles per hour. What was his average speed for the entire trip?

And, no, your first guess was most likely not correct.

12. The Year in Numbers

A moldy oldie, to be sure, but this puzzle is always a crowd-pleaser for those who haven’t seen it before.

Use the digits of the new year — 2, 0, 1, and 9 — and any mathematical operations to form the integers from 1 to 100. For instance, you can form 1 as follows: 2 × 0 × 9 + 1 = 1.

For an added challenge, add the restriction that you must use the four digits in order.

Bonus: A Book

My sons get sick when they read in the car. That’s why I love the two books Without Words and More Without Words by James Tanton. Literally, there are no words! Each puzzle is presented using a few examples, and then students must follow the same rules to solve a few similar, but more challenging, puzzles.

Without Words

Wherever you’re headed during the holiday break — driving to your relatives’ house, flying to Fort Lauderdale, or just relaxing at home — I hope your holidays are filled with joy, happiness, and lots of math!

December 21, 2018 at 8:45 am Leave a comment

One-Letter Quiz

The answer to each question below is a letter of the alphabet. Each letter is used exactly once. (Thanks for the idea, Ask Me Another.) Good luck!

Want to amuse your friends, irritate your students, or annoy people you’ve just met? Download a PDF version of the One-Letter Quiz (without answers).

  1. The letter used to represent the square root of -1.
  2. This letter is often added to indefinite integrals to show that any function with at least one antiderivative has an infinite number of them.
  3. The most frequently occurring letter in English words.
  4. The letter most recently added to the modern, 26-letter English alphabet.
  5. The letter represented by four dots in Morse Code.
  6. A type of road intersection with three arms.
  7. Although long out of use, this letter was used in the middle ages as the Roman numeral to represent 90.
  8. This letter is used for the temperature scale in which the boiling point is 212 degrees and the freezing point is 32 degrees.
  9. The most common blood type.
  10. The rating from the Motion Picture Association of America that requires children under 17 to be accompanied by an adult.
  11. The 43rd President of the United States.
  12. The only vowel that does not appear in the spelling of any single-, double-, or triple-digit numbers.
  13. Behind s and c, the third most common letter with which English words begin.
  14. With plan, the letter used to refer to a typically less desirable alternative.
  15. The Roman numeral for 500.
  16. The symbol for potassium on the periodic table.
  17. The most common variable in algebra.
  18. The Roman numeral for 5.
  19. The “score” used to indicate the number of standard deviations a data point is from the mean.
  20. The letter commonly used to refer to the vertical axis on a coordinate graph.
  21. Although every adult can recognize the loop-tail version of this lowercase letter in print, less than one-third of participants in a Johns Hopkins study could correctly pick it out of a four-option lineup.
  22. The clothing size that increases when preceded by an X.
  23. The shape of the “happiness curve,” which implies that most people are least happy in their 50’s.
  24. The shape of a logistic growth curve, which increases gradually at first, more rapidly in the middle, and slowly at the end, leveling off at a maximum value after some period of time.
  25. The only letter that does not appear in the name of any US state.
  26. The answer to the riddle, “It occurs once in a minute, twice in a moment, but never in a thousand years.”

Answers (and Notes of Interest)

  1. I
  2. C
  3. E
  4. J : in 1524, Gian Giorgio Trissino made a clear distinction between the sounds for i and j, which were previously the same letter
  5. H
  6. T
  7. N : see Wikipedia for a list of other Roman numerals used in medieval times
  8. F
  9. O
  10. R
  11. W : should probably be “Dubya” instead of “Double U,” but whatever
  12. A
  13. P : as you might expect, more English words start with S than any other letter; based on the ENABLE word list, P is the second most common initial letter, followed by C
  14. B
  15. D
  16. K : the symbol K comes from kalium, the Medieval Latin for potash, from which the name potassium was derived
  17. X
  18. V
  19. Z
  20. Y
  21. G : a lowercase g can be written in two different ways, and the more common version in typesetting (known as the “loop-tail g“) can be recognized but not written by most adults, as recounted on the D-Brief blog
  22. L
  23. U : see this article from The Economist, especially this image
  24. S
  25. Q
  26. M



September 4, 2018 at 7:23 am Leave a comment

WODB, Quora Style

The following puzzle was recently posted on Quora:

Which of the following numbers don’t belong: 64, 16, 36, 32, 8, 4?

What I liked about this puzzle was the answer posted by Danny Mittal, a sophomore at the Thomas Jefferson High School for Science and Technology. Danny wrote:

64 doesn’t belong, as it’s the only one that can’t be represented by fewer than 7 binary bits.

36 doesn’t belong, as it’s the only one that isn’t a power of 2.

32 doesn’t belong, as it’s the only one whose number of factors has more than one prime factor.

16 doesn’t belong, as it’s the only one that can be written in the form xy, where x is an integer and y is a number in the list.

8 doesn’t belong, as it’s the only one that doesn’t share a digit with any other number in the list.

4 doesn’t belong, as it’s the only one that’s a factor of all other numbers in the list.

I suspect that Danny has visited Which One Doesn’t Belong or has read Christopher Danielson’s Which One Doesn’t Belong. Or maybe he’s just a math teacher groupie and trolls MTBoS.

But then Jim Simpson pointed out the use of “don’t” in the problem statement, which I had assumed was a grammatical error. Jim interpreted this to mean that there must be two or more numbers that don’t belong for the same reason, and with that interpretation, Jim suggested the answer was 32 and 8, since all of the others are square numbers.

Don’t get me wrong — I don’t think this is a great question. But I love that it was interpreted in many different ways. It could lead to a good classroom conversation, and it makes me consider all sorts of things, not the least of which is standardized assessments. How many times have students gotten the wrong answer for the right reason, because they interpreted an item on a state exam or the SAT differently than the author intended? And how many times have we bored students with antiseptic questions, only because we knew they’d be free from such alternate interpretations? Both scenarios make me sad.

January 6, 2018 at 8:47 am Leave a comment

2017 KenKen International Championship

If you like puzzles and ping pong, then Pleasantville, NY, was the place to be on December 17.

More than 200 Kenthusiasts — people who love KenKen puzzles — descended on Will Shortz’s Westchester Table Tennis Center for the 2017 KenKen International Championship (or the KKIC, for short). Participants followed 1.5 hours of solving KenKen puzzles with a pizza party and several hours of table tennis.

KenKen Logo

The competition consisted of three rounds, with the three puzzles in each round slightly larger and more difficult than those from the previous round. Consequently, competitors were given 15, 18, and 20 minutes to complete the puzzles in the first, second, and third rounds, respectively.

Competitors earned 1,000 points for each completely correct puzzle, and 0 points for an incomplete or incorrect puzzle. In addition, a bonus of 5 points was earned for every 10 seconds in which a puzzle was turned in before time was called. So, let’s say you got two of the three puzzles correct and handed in your answers with 30 seconds remaining in the round; then, your score for that round would be

2 \times 1000 + \frac{30}{10} \times 5 = \bf{2{,}015}

The leader after the written portion was John Gilling, a data scientist from Brooklyn, whose total score was 10,195. And if you’ve been paying attention, then you know what that means — Gilling earned 9,000 points for completing all of the puzzles correctly, so his time bonus was 1,195 points… which is the amount you’d earn for turning in the puzzles 2,390 seconds (combined) before time was called. The implication? Gilling solved all 9 puzzles from the written rounds — which contained a mix of puzzles from size 5 × 5 to 8 × 8 — in just over 13 minutes.


As a result, Gilling, the defending champion, earned a spot in the Championship Round against Tess Mandell, a math teacher from Boston; Ellie Grueskin, a high school senior at The Hackley School; and Michael Holman, a technology consultant. In the final round, each of them attempted a challenging 9 × 9 puzzle, which was displayed on an easel for the crowd to see. Solving a challenging 9 × 9 is tough enough; having to do it as 200 kenthusiasts follow your every move is even tougher.

So, how’d they do? See for yourself…

When the dust settled, Gilling had successfully defended his title. For his efforts, he received a check for $500. But more importantly, he retained bragging rights for one more year.

John Gilling - 2017 KKIC

John Gilling and his winning KenKen board at the 2017 KKIC

If you think you’ve got what it takes to compete with the best KenKen solvers, try your hand at the 9 × 9 puzzle that was used in the final round. In the video above, you saw how fast Gilling solved it to win the gold. But even the slowest of the four final-round participants finished in under 15 minutes.

Again, wow.

Finally, I’d be failing as a father if I didn’t mention that my sons Alex and Eli competed in the Delta (age 10 and under) division. Though bested by Aritro Chatterjee, a brilliant young man who earned a trip to the 2017 KKIC by winning the UAE KenKen Championship, Eli took the silver, and Alex brought home the bronze. They’re shown in the photos below with Bob Fuhrer, the president of Nextoy, LLC, the KenKen company and host of the KKIC.

Eli and Bob Fuhrer - KKIC    Alex and Bob Fuhrer - KKIC


For more KenKen puzzles, check out, or see my series of posts, A Week of KenKen.

December 26, 2017 at 9:10 am 2 comments

Is Your Gödel Too Tight?

I don’t care what Stevie Nicks says, thunder does not only happen when it’s raining. And sorry, Kelly Clarkson, I’m not standing at your door because I’m sorry.

Logical fallacies are rampant in song lyrics. (Don’t even get me started.) I’m therefore hopeful that you won’t attempt to channel your inner songwriter while trying to solve the following logic puzzles, arranged roughly in order of difficulty.

Here’s Looking at You

Jack is looking at Anne, and Anne is looking at George. Jack is married, George is not. Is a married person looking at an unmarried person?

Beer is Proof that God Loves Us

Three people walk into a bar, and the bartender asks, “Would all of you like a beer?” The first says, “I don’t know.” The second says, “I don’t know.” The third emphatically replies, “Yes!”

Why was the third one able to respond in the affirmative?

Five to the Third

A five-digit number is equal to the sum of its digits raised to the third power. Alphametically,

CUBED = (C + U + B + E + D)3

What is the five-digit number?

Martin Gardner’s Children

I ran into an old friend, and I asked about her family. “How old are your three kids now?”

She said the product of their ages was 36. I replied, “Sorry, I still don’t know how old they are.”

She then said, “Well, the sum of their ages is the same as the house number across the street.”

“I’m sorry,” I said. “I still don’t know how old they are.”

Finally, she told me that the oldest one has red hair, and I finally realized their ages.

How old are my friend’s children?

If At First You Don’t Succeed…

If you take a positive integer, multiply its digits to obtain a second number, multiply all of the digits of the second number to obtain a third number, and so on, the persistence of a number is the number of steps required to reduce it to a single-digit number by repeating this process. For example, 77 has a persistence of four because it requires four steps to reduce it to a single digit: 77-49-36-18-8. The smallest number of persistence one is 10, the smallest of persistence two is 25, the smallest of persistence three is 39, and the smaller of persistence four is 77.

What is the smallest number of persistence five?

The Hardest Logic Puzzle Ever

Three gods A, B, and C are called, in no particular order, True, False, and Random. True always speaks truly, False always speaks falsely, but whether Random speaks truly or falsely is a completely random matter. Your task is to determine the identities of A, B, and C by asking three yes-no questions; each question must be put to exactly one god. The gods understand English, but will answer all questions in their own language, in which the words for yes and no are da and ja, in some order. You do not know which word means which.

(This puzzle is attributed to Raymond Smullyan, but the twist of not knowing which word means which was apparently added by computer scientist John McCarthy.)

October 26, 2017 at 6:32 am Leave a comment

Number Challenge from Will Shortz and NPR

Typically, the NPR Sunday Puzzle involves a word-based challenge, but this week’s challenge was a number puzzle.

[This challenge] comes from Zack Guido, who’s the author of the book Of Course! The Greatest Collection of Riddles & Brain Teasers for Expanding Your Mind. Write down the equation

65 – 43 = 21

You’ll notice that this is not correct. 65 minus 43 equals 22, not 21. The object is to move exactly two of the digits to create a correct equation. There is no trick in the puzzle’s wording. In the answer, the minus and equal signs do not move.

Seemed like an appropriate one to share with the MJ4MF audience. Enjoy!

October 18, 2017 at 8:07 am 2 comments

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About MJ4MF

The Math Jokes 4 Mathy Folks blog is an online extension to the book Math Jokes 4 Mathy Folks. The blog contains jokes submitted by readers, new jokes discovered by the author, details about speaking appearances and workshops, and other random bits of information that might be interesting to the strange folks who like math jokes.

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