Here is a nice logic puzzle from 2014 Futility Closet.
“Only one of these statements is true. Which is it?
_________A. All of the below
_________B. None of the below
_________C. One of the above
_________D. All of the above
_________E. None of the above
_________F. None of the above”
See Pointing Fingers for solutions.
For a change of pace, here is an early puzzle from Alex Bellos in The Guardian.
“Happy New Year guzzlers! Today’s first problem concerns squirrels. Have a nibble—it’s not too hard a nut to crack.
The Squirrel King has buried the Golden Acorn beneath one of the squares in this 6x6 grid. Three squirrels—Black, Grey and Red—are each standing on a square in the grid, as illustrated.
(Note: for the purposes of today, squirrels can speak, hear, read, count and are perfect logicians. They can also move in any direction horizontally and vertically, not just the direction these cartoons are facing. They all can see where each other is standing, and the cells in the grid are to be considered squares.)
The Squirrel King hands each squirrel a card, on which a number is written. The squirrels can read only the number on their own card. The King tells them: ‘Each card has a different number on it, and your card tells you the number of steps you are from the square with the Golden Acorn. Moving one square horizontally or vertically along the grid counts as a single step.’ (So if the acorn was under Black, Black’s card would say 0, Grey’s would say 4, and Red’s 5. Also, the number of steps given means the shortest possible number of steps from each squirrel to the acorn.)
The King asks them: ‘Do you know the square where the Golden Acorn is buried?’ They all reply ‘no!’ at once.
Red then says: ‘Now I know!’
Where is the Golden Acorn buried? …”
See the Squirrel Puzzle
Yet another interesting problem from Presh Talwalkar.
“Two side-by-side squares are inscribed in a semicircle. If the semicircle has a radius of 10, can you solve for the total area of the two squares? If no, demonstrate why not. If yes, calculate the answer.”
This puzzle shares the characteristics of all good problems where the information provided seems insufficient.
See the Sum of Squares Puzzle for solutions.
Here is a collection of puzzles from the great logic puzzle master Raymond Smullyan in a “Brain Bogglers” column for the 1996 Discover magazine.
See Family Values for solutions.
This is a problem from the UKMT Senior Challenge for 2019. (It has been slightly edited to reflect the colors I added to the diagram.)
“The edge-length of the solid cube shown is 2. A single plane cut goes through the points Y, T, V and W which are midpoints of the edges of the cube, as shown.
What is the area of the cross-section?
A_√3_____B_3√3_____C_6_____D_6√3_____E_8”
See Another Cube Slice Problem for solutions.
This is a nice Brainteaser from the Quantum math magazine.
“Line segment MN is the projection of a circle inscribed in a right triangle ABC onto its hypotenuse AB. Prove that angle MCN is 45°.”
See the Circle Projection Problem.
Here is another problem (slightly edited) from the Sherlock Holmes puzzle book by Dr. Watson (aka Tim Dedopulos).
“Holmes and I were walking along a sleepy lane in Hookland, making our way back to the inn at which we had secured lodgings after scouting out the estates of the supposed major, C. L. Nolan. Up ahead, a team of horses were slowly pulling a chained tree trunk along the lane. Fortunately it had been trimmed of its branches, but it was still an imposing sight.
When we’d overtaken the thing, Holmes surprised me by turning sharply on his heel and walking back along the trunk. I stopped where I was to watch him. He continued at a steady pace until he’d passed the last of it, then reversed himself once more, and walked back to me.
‘Come along, old chap,’ he said as he walked past. Shaking my head, I duly followed.
‘It took me 140 paces to walk from the back of the tree to the front, and just twenty to walk from the front to the back,’ he declared.
‘Well of course,’ I said. ‘The tree was moving, after all.’
‘Precisely,’ he said. ‘My pace is one yard in length, so how long is that tree-trunk?’
Can you find the answer?”
See the Tree Trunk Puzzle for solutions.
I found an interesting geometric statement in a paper of Glen Van Brummelen cited in the online MAA January 2020 issue of Convergence:
“For instance, Abū’l-Wafā’ describes how to embed an equilateral triangle in a square, as follows: extend the base GD by an equal distance to E. Draw a quarter circle with centre G and radius GB; draw a half circle with centre D and radius DE. The two arcs cross at Z. Then draw an arc with centre E and radius EZ downward, to H. If you draw AT = GH and connect B, H, and T, you will have formed the equilateral triangle.”
So the challenge is to prove this statement regarding yet another fascinating appearance of an equilateral triangle.
See the Triangle of Abū’l-Wafā’
This turned out to be a challenging geometric problem from Poo-Sung Park posted at the Twitter site #GeometryProblem
“Geometry Problem 92: What is the ratio of a:b?”
See the Envelope Puzzle for solutions.
Here is another problem from the Quantum magazine, only this time from the “Challenges” section (these are expected to be a bit more difficult than the Brainteasers).
“A quadrangle is inscribed in a parallelogram whose area is twice that of the quadrangle. Prove that at least one of the quadrangle’s diagonals is parallel to one of the parallelogram’s sides. (E. Sallinen)”
