Imprisoned Logicians

Two friends, logicians – Ein and Stein – get imprisoned in two distant cells in a castle. Both cells have just one door, and a window with 8 bars in the first cell, and 12 bars in the second cell. The first day both logicians get the same letter from the prison master:

“The total number of bars in the two prison cells in this castle is either 18 or 20. Starting tomorrow, every morning I will go first to Ein and then to Stein, and will ask how many bars the other logician has. If one of you answers correctly, I will immediately let both of you leave the castle. If one of you answers incorrectly, I will execute both of you. Of course, you can always decide not to answer and just stay imprisoned.
I have sent a copy of this letter to you and your friend. There is no point in trying to communicate with him – your cells are far away from each other, and he won’t hear you.”

Will the logicians manage to escape the castle eventually? When will they do it?

Solution coming soon.


Lost In the Forest

You are lost in the middle of a forest, and you know there is a straight road exactly 1 km away from you, but not in which direction. Can you find a path of distance less than 640 m which will guarantee you to find the road?

Imagine there is a circle with a radius of 100 m around you, and you are at its center O. Let the tangent to the circle directly ahead of you be t. Then, follow the path:

  1. Turn left 30 degrees and keep walking until you reach the tangent t at point A for a total of 100×2√3/3 meters, which is less than 115.5 meters.
  2. Turn left 120 degrees and keep walking along the tangent to the circle until you reach the circle at point B for a total of 100×√3/3 which is less than 58 meters.
  3. Keep walking around the circle along an arc of 210 degrees until you reach point C for a total of 100×7π/6 which is less than 366.5 meters.
  4. Keep walking straight for 100 meters until you reach point D on the tangent t.

Spot the James Bond Film

Can you spot the five James Bond films?

The films are:

  1. “Diamonds are Forever” (Diamonds + R + Four + Heifer)
  2. “Moonraker” (Moon + Rake + R)
  3. “Octopussy” (Octopus + E)
  4. “Golden Eye” (Gold + Hen + Eye)
  5. “The World Is Not Enough” (The World + E’s + Knot + N + Oeuf)

This rebus is taken from the book “A Collection of Spots”. Inside the book you will find 48 more puzzles.


Beautiful Geometry 1

Since 2018, Catriona Shearer, a UK teacher, has been posting on her Twitter various colorful geometry puzzles. In this mini-course, we cover some of her best problems and provide elegant solutions to them. Use the pagination below to navigate the puzzles, which are ordered by difficulty.

Example Problems

What fraction of the hexagon is shaded?
Which area is larger, the yellow or the red one?
What is the area of the large square?
What fraction of the outer circle is shaded?
If the circle radius is 1, what area is shaded?
What fraction of the decagon is shaded?

Puzzle Tournament 4

Puzzle Prime’s fourth puzzle tournament was organized on February 26, 2022. Congrats to REDCROUTONS who solved all the puzzles (even found a small mistake in one of them)!

You have 60 minutes to solve 6 puzzles, each worth 1 point. Upload your solutions as a pdf, document, or image, using the form below. Good luck!

Time for work: 1 hour

Each problem is worth 1 point. Use the form at the bottom of the post to send your solutions.

1. The Grid

by Puzzle Prime

Figure out how the last portion (7×5 in yellow) of the grid should be colored in black and white.

2. Hexado

by Dr. DJ Upton

Place arrows along hexagon edges so that the number of arrows pointing to each hexagon equals the number of dots inside, adhering to the following rules:

  1. Arrows cannot be touching.
  2. Arrows cannot be placed on dashed edges.

3. Segments

by Puzzle Prime

Use at most 27 segments to create the largest number with distinct digits.

Notes: For example, the number 273914 would use 5+3+5+6+2+4=25 segments.

4. Constellations

by Raindrinker

Connect the stars with lines, so that the number inside each star corresponds to the number of lines connected to it, and the number outside each star corresponds to the total number of stars in its group.

Note: No line connecting two stars can pass through a third star.


5. Chess Connect

by Puzzle Prime

The starting and ending positions of 6 chess pieces are shown on the board. Find the trajectories of the pieces, if you know that they do not overlap and completely cover the board.

Notes: The pieces can not backtrack. Two trajectories can intersect diagonally but can not pass through the same square. Only the Knight has a discontinuous trajectory.


6. Broken Square

by Puzzle Prime

Use exactly 5 out of these 16 pieces to build a 7×7 grid, without overlapping.

Note: You can rotate the pieces, but you cannot mirror them.


A Broken Circle

There are N points on a circle. If we draw all the chords connecting these points and no three of them intersect at the same point, in how many parts will the interior of the circle get broken?

For example, when N is equal to 1, 2, 3, 4, and 5, we get 1, 2, 4, 8, and 16 parts respectively.

The answer, somewhat surprisingly, is not 2ᴺ⁻¹, but 1 + N(N-1)/2 + N(N-1)(N-2)(N-3)/24.

In order to see that, we start with a single sector, the interior of the circle, and keep successively drawing chords. Every time we draw a new chord, we increase the number of parts by 1 and then add 1 extra part for each intersection with previously drawn chords.

Therefore, the total number of parts at the end will be:

1 + the number of the chords + the number of the intersections of the chords

Each chord is determined by its 2 endpoints and therefore the number of chords is N(N-1)/2.

Each intersection is determined by the 4 endpoints of the two intersecting chords and therefore the number of intersections is N(N-1)(N-2)(N-3)/4!.