Category Archives: Puzzles and Problems

River Traffic Problem

Here is another elegant Quantum math magazine Brainteaser problem.

“A raft and a motorboat set out downstream from a point A on the riverbank. At the same moment a second motorboat of the same type sets out from point B to meet them. When the first motorboat arrives at B, will the raft (floating with the current) be closer to point A or to the second motorboat? (G. Galperin)”

See the River Traffic Problem

Ubiquitous Radius Problem

This is an interesting problem from Posamentier and Lehmann’s Mathematical Curiosities.

“In the figure we have a semicircle with the point P randomly placed on the diameter. Points A and B are situated on the circle such that they form angles of 60° with the diameter as shown in the figure. This problem asks us to show that the length of AB is equal to the radius of the semicircle.”

See the Ubiquitous Radius Problem

Mystery of the Dancing Men

Manmohan Kaur took Arthur Conan Doyle’s popular 1903 Sherlock Holmes story “The Adventure of the Dancing Men” and used it in his math classes to illustrate the logic and mathematics involved in solving codes and ciphers.  I thought his idea might work as a puzzle.  It has been years since I read the story, so I had forgotten the decryption and found it quite doable from the setup provided by Kaur.  Here is his presentation, subject to further edits and reductions in size on my part.

“The original story has been shortened and simplified. Reference to England has been completely removed and some other superfluous information that distracts the reader instead of helping solve the mystery have been omitted. In the original story Elriges is the name of an inn but we have taken the liberty to use it loosely as the name of a town.

The pictures of all stick figure messages except the fourth are from the collection The Return of Sherlock Holmes. The original story has a typographical error that throws off the decryption scheme. To remove this (intentional or unintentional) error, the fourth figure has been taken from Trap and Washington’s Introduction to Cryptogra­phy with Coding Theory. The fourth message is meant to have a different handwriting, so this serves our purposes well.

Condensed Story

Hilton Cubitt of Elriges visits you and gives you a paper with the following mysterious sequence of stick figures that he found lying on the sun-dial in his mansion.

Message 1:

Cubitt explains that he recently married a Chicago woman named Elsie Patrick. Before the wedding, she had asked him never to ask about her past, as she had had some “very disagreeable associations” in her life, although she said that there was nothing that she was personally ashamed of. Their mar­riage had been a happy one until the messages began to arrive, first mailed from Chicago and then appearing in the garden of his mansion.

The messages had made Elsie very afraid but she did not explain the reasons for her fear, and Cubitt insisted on honoring his promise not to ask about Elsie’s life in Chicago. You look at the figures closely to understand them a little better and notice that some of the figures are holding flags. What could the flags mean? Perhaps the end of words?

The next morning Cubitt finds “a fresh crop of dancing men drawn in chalk upon the black wooden door of the tool-house”:

Message 2:

Two mornings later, “a fresh inscription had appeared”:

Message 3:

Three days later, “a message was left scrawled upon paper, and placed under a pebble upon the sun-dial”:

Message 4:

Cubitt gives copies of all these messages to you. Your task is to help him understand what is going on. You call your friend in the Chicago Police Department and ask her to find background information on Elsie Patrick. You learn that Elsie is the daughter of a Chicago crime boss, and was engaged to Abe Slaney, who worked for her dad, and that she had fled to escape her old life.

You examine all the occurrences of the dancing figures. Message 4 is in a different handwriting, so you guess that it is from a different person, most likely, Elsie, while messages 1, 2 and 3 are from the unknown person (the criminal). You spend the next two days trying to make some sense of the stick figures. You are now sure that the flags on some of the figures indicate the end of words. You also know that a simple substitution cipher is being used for the encryption, and that frequency analysis is the way to solve these ciphers.

Three days later, another message appears.

Message 5:

This message causes you to fear that the Cubitts are in immediate danger. You rush to Elriges and find Cubitt dead of a bullet to the heart and his wife gravely wounded from a gunshot to the head. What do the messages say?

Inspector Martin of the Norfolk Constabulary believes that it is a murder-suicide attempt; Elsie is the prime suspect. But you, after noting some inconsistencies in that theory, know that there is a third person involved. How will you prove to Inspector Martin that a third person is involved?”

See The Mystery of the Dancing Men

Hard Time Conundrum

This problem comes from the “Problems Drive” section of the Eureka magazine published in 1955 by the Archimedeans at Cambridge University, England.  (“The problems drive is a competition conducted annually by the Archimedeans. Competitors work in pairs and are allowed five minutes per question ….”)

“There are ten times as many seconds remaining in the hour as there are minutes remaining in the day. There are half as many minutes remaining in the day as there will be hours remaining in the week at the end of the day.  What time is it on what day?”

One of the hardest parts of the problem is just being able to translate the statements into mathematical terms.  Solvable in 5 minutes?!!!

See the Hard Time Conundrum

Tethered Triangle Puzzle

This is another problem from MEI’s MathsMonday.

“Two equilateral triangles share a common vertex. Show that the lengths marked a and b are equal for any such arrangement.”

This seems quite amazing at first.  One can picture the small triangle swinging back and forth with red bungee chords tethering its bottom vertices to the bottom vertices of the large triangle.  It would seem remarkable that the lengths of the chords would remain equal to each other throughout.

See the Tethered Triangle Puzzle

Wandering Epicycle

Here is an intriguing problem from the 2021 Math Calendar.

“If the smaller circle of diameter 7 rotates without slipping within the larger circle, what is the length of the path of P?”

The problem did not state clearly how far the smaller circle should rotate.  Its answer implied it should complete just one full (360°) rotation within the larger circle.

Recall that all the answers are integer days of the month.

See the Wandering Epicycle

(Update 1/3/2022)  First, this problem is dealt with in more detail and more expansively on the Mathologer Youtube website by Burkard Polster in his 7 December 2018 post on the “Secrets of the Nothing Grinder” (Figure 1).  A further, deeper discussion of epicycles is given in the Mathologer’s 6 July 2018 post on “Epicycles, complex Fourier and Homer Simpson’s orbit” (Figure 2).  And finally, a panoply of related puzzles is given in the 30 December 2021 Mathologer post “The 3-4-7 miracle. Why is this one not super famous” (Figure 3).

This last post reveals the ambiguity of the idea of “one full (360°) rotation” I disingenuously added to the problem to try to get the answer given in Math Calendar version.

For a complete explanation see the Wandering Epicycle Addendum.

Polygon Rings

This is a nice geometric problem from the Scottish Mathematical Council (SMC) Senior Mathematical Challenge of 2008.

“Mahti has cut some regular pentagons out of card and is joining them together in a ring. How many pentagons will there be when the ring is complete?

She then decides to join the pentagons with squares which have the same edge length and wants to make a ring as before. Is it possible? If so, determine how many pentagons and squares make up the ring and if not, explain why.”

See the Polygon Rings

MoMath Mazes

I found these mazes on Twitter and thought they might make a relaxing puzzle interlude.  They come from photographs of the street in front of the Museum of Mathematics (MoMath) in New York.   The idea is to traverse the mazes from the Start to the Goal making only right turns.  It was difficult working out the pattern of the green maze, especially the upper right corner.

See the MoMath Mazes

Puzzles and Problems: MoMath