You are given two ropes and a lighter. Each of the two ropes has the following property: if you light one end of the rope, it will take exactly one hour to burn to the other end. It doesn’t necessarily burn at a uniform rate. How can you measure a period of 45 minutes?
2. Cork, Bottle, Coin
You put a coin in an empty bottle and insert a cork in the bottle’s opening. How can you remove the coin without taking out the cork or breaking the bottle?
3. A Locked Door
You have 100 bags of coins each with 100 coins, but only one of these bags has gold coins. The gold coin weighs 1.01 ounce and the other coins weighs 1 ounce. You also have a scale, but can only use it once. How can you identify the bag of gold coins?
4. Glass Half Full
You are in an empty room with a transparent glass of water. The glass is a right cylinder and appears to be half full. How can you accurately figure out whether the glass is half full, more than half full, or less than half full? You have no rulers or writing utensils?
5. Globe Traversal
How many places are there on earth where one could walk one mile south, then one mile west, then one mile north and end up in the same spot? Assume the earth is a solid smooth sphere?
6. Brown Eyes and Red Eyes
There is an island of monks where everyone has either brown eyes or red eyes. Monks who have red eyes are cursed, and are supposed to commit suicide at midnight. However, no one ever talks about what color eyes they have, because the monks have a vow of silence. Thus, no one knows their own eye color; they can only see the eye colors of other people, and not mention them. Every day, the monks enjoy a silent brunch together at a round table. One day, a tourist visits the island monastery, and, unaware that he’s not supposed to talk about eyes, says “At least one of you has red eyes.” Having acquired this new information, something dramatic happens among the monks. What happens?
7. Row, Row, Row Your Trees
How can seven trees be planted such that there are six rows of trees in straight lines consisting of three trees?
8. Unusual Suspects
The police rounded up Jim, Bud and Sam yesterday, because one of them was suspected of having robbed a bank. The three suspects made the following statements under intensive questioning: Jim: I’m innocent. Bud: I’m innocent. Sam: Bud is guilty. If only one of these statements is true, who robbed the bank?
9. Infinite Quarter Sequence
You are wearing a blindfold and thick gloves. An infinite number of quarters are laid out on a table of infinite area. 20 of these quarters are tails and the rest are heads. How can you can split the quarters into 2 piles where the number of tails quarters is the same in each? You are allowed to move the quarters and to flip them, but you can never tell what state a quarter is currently in.