100, 10, 1
Your friend, Cole, shows you his first attempt at programming: a calculator that takes in a binary sequence and does an arbitrary computation on it: If the...
Two Children 2
You’re talking to a different colleague at work when it comes up in the conversation that he has 2 children. “What genders are they?” you ask again. “Well, the...
Two Children 1
You’re talking to your colleague at work when it comes up in the conversation that he has 2 children. “What genders are they?” you ask. “Well, I have at least...
Two-Bullet Russian Roulette
After offending the toughest guy, Big Billy, at the saloon, he challenges you to a game of Russian Roulette. He takes out a revolver with 6 chambers and loads...
27 Cubes
A 9cm cube is to be cut into 27 smaller cubes, each of side length 3cm. This can be done with 6 straight cuts to the original cube: 2 parallel cuts along each...
The Keyboard Prank
Anne comes into the office to find that her co-workers had played a prank on her. The keycaps on her keyboard had been shuffled such that if she types out a...
One Less Than a Prime Squared
Show that p^2-1 is always a multiple of 24 if p is a prime that is bigger than 3.
Confused Bank Teller
When Dorothy went to cash in a cheque, the confused bank teller swapped the dollars and cents. He gave her dollars in place of cents, and cents in place of...
Watermelons Drying in The Sun
A farmer is proud of his newest feat: growing a 120 kg watermelon that he wants to enter in the farming contest next week. Using a special sensor, he detects...
Two Girlfriends and Two Trains
Unfaithful Bob has two girlfriends Alice and Carol. Bob can take a train ride from the city’s train station to get to them. There is only 1 line going in both...
Fastest 3 in 25 Horses
You visit a farm, hoping to buy the 3 fastest horses they have. The farmer says he has 25 horses, but he has no idea how fast each of them are. Instead, he...
Covering a Chessboard 1
You’re given an 8x8 chessboard with 2 opposite corner tiles removed. You can place dominoes on the board, with each one covering exactly 2 adjacent tiles (you...
Hummingbird
A train leaves Oakridge at 20 km/h, headed for Fairhaven. At the same time, another train leaves Fairhaven at 30 km/h, headed for Oakridge on the same track....
Close Shave with a Train
Alan is traversing a railway track that spans over a treacherous ravine. As he reaches \frac{3}{8} of the way across the track, he hears the whistle of a train...
The Camel Race
An aging sheikh, nearing the end of his life, decides to leave his vast fortune to one of his two sons. To determine the heir, he devises a strange contest...
Taking Pills
After falling ill, Josh sees a doctor and is prescribed some medication. He is to take 1 pill of Xenoril and 1 pill of Yvexol every 24 hours. The two pills...
Proving Rules on Cards
On the table are 4 cards, each one labelled “A”, “D”, “3”, and “6” respectively. You know that every card has a letter on one side and a number on the other. I...
Marble Pairs
You're given a jar containing 100 marbles, each one coloured either black or white, and there is at least 1 marble of each colour. You notice that if you pick...
Guessing Hats 1
Angry with his three wise men, the evil king subjects them to an impossible task: they’ll have to guess the colour of the hats on their heads. If any one of...
Counting Towns from a Tower
The evil king, no longer trusting his 2 wise men, wants to test their wisdom, so he locks them in a tall tower in the middle of his kingdom. One wise man is...
Traversing the Hypercube
Ossarion-429 is a multi-dimensional being travelling across hypercubes of various dimensions. Bored and having nothing else to do, he wonders to himself if...
Diagonals on a Cube
Two diagonal lines are drawn on a cube. What is the angle between these two lines?
Average Salaries 2
Back in Average Salaries 1, 12 employees came up with a method to find out their average salary without revealing their individual salaries to anyone. Here's...
Average Salaries 1
There's a global pandemic going on, so everyone is working from home. During this time, 12 employees with similar roles wonder if there are any large salary...
Chain Link 2
An apprentice is working for you, the village’s master blacksmith, for 21 days. For payment, you have agreed to give him a gold chain with 21 links, one link...
Chain Link 1
Five apprentices are working for you, the village’s master blacksmith. After three days of work, you now have to pay them each a gold chain of 3 links. You...
Coins on a Table
You and a friend want to settle who is the smarter of you two with a game. The game is played on a circular table, with you and your friend taking turns to put...