With a background in logic and math, I love riddles, although 'logic puzzle' might be more precise. All of the riddles here rely on sound logical thinking rather than word tricks. Hopefully I can remember more of these in the future. No answers posted here, anybody reading this is free to email me though.
Note: I don't think I've come up with any of these on my own, they are all being re-told without express written consent.
Difficulty: easy
You have in front of you a large jar of black and white jelly beans. You repeatedly reach in and pull out two random beans. If you pull out two beans of the same color, you put one white bean back in, if they are a different color, you put one black bean back in.
As you repeat this process, eventually there is only one bean left in the jar. What color will it be? More specifically, what is the minimum amount of information you will need about the contents of the jar in order to determine the color of the final bean?
Difficulty: medium
6 scientists are collaborating on experiments for which they need access to a vault with highly enriched uranium. However, none of the scientists is considered trustworthy. The scientists agree that if a majority of them (4 or more) are present, they should be able to open the vault, but 3 or fewer should not be able to. To achieve this, they can use as many ordinary locks as they want, and for each lock can make as many copies of the key as they want. The solution will involve using a number of locks, and giving each lock's keys to some subset of the scientists.
How many locks must be used so that no group of three scientists can unlock all of them, but any group of four or more can? How are keys for the locks distributed?
Difficulty: medium
100 monks live in a monastery. They follow a strict code of no communication, verbal or otherwise. One night they are all told that an unknown number of them have not been faithful and must commit suicide. The doomed monks will wake up the next morning with an 'X' on the foreheads.
Unfortunately, they have no mirrors and thus no way to know if they are marked. They do, however, all gather for dinner each night, at which they can see which of the other monks is marked, but of course, they cannot communicate this. The doomed monks must kill themselves in their room at night.
How can the doomed monks figure out that they are marked? After how many days will all of them have killed themselves?
Difficulty: hard
A banker receives 12 coins, one of which is counterfeit and might be heavier or lighter than the 11 legitimate coins. Using a balance scale, and exactly 3 weighings, he can determine exactly which coin is counterfeit, and whether it is heavier or lighter than the good coins.
How is this done? A solution should include a decision tree of exactly which coins are weighed at each step, contingent on the previous steps. Unfortunately, he solution is very difficult, it does not have a simple description.
This riddle can be generalized to N total coins, C counterfeit coins, but as far as I know there isn't an elegant solution for the minimum numbe of weighings.
Difficulty: hard
11 prisoners are being held by a malicious warden. He offers them a game to earn their freedom. In an empty room, he has two large levers, each of which has a clear 'up' and 'down' position, but which otherwise have no function. The warden will repeatedly pick inmates as he pleases and take them to the lever room, upon which they must switch the position of exactly one of the levers.
The prisoners are allowed to meet for one hour to discuss strategy, but then they will be solitarily confined in their cells and have no communication except through the position of the levers. Their goal is to determine when each inmate has been to the lever room at least twice. If one prisoner can say this with certainty upon entering the lever room, the prisoners will all be freed. There is no penalty for making this claim late as long as it is correct. However, they will all be executed if the claim is made too early.
The warden can observe the prisoner's strategy meeting, and thus can take the prisoners into the room in any order which he thinks will foil their strategy. He is also free to set them to any start state he wishes, although after that he will not touch the levers himself.
What strategy can the prisoners use to eventually state with certainty that all have visited the lever room twice?