# Questions tagged [keys-and-locks]

For puzzles about using keys to open locks and doors.

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17k views

### 10 Prisoners, 10 Keys, and 7 Years

One day, the warden of a prison is, like most wardens in puzzles, feeling a little bit capricious and decides that he wants to get rid of his prisoners, one way or another. He gathers all the ...
2k views

### One Hundred Lockboxes of Wood and Steel

A bit beyond perceptions reach, I sometimes believe I see that life is two locked boxes, each containing the other’s key. ― Piet Hein You have one hundred lockboxes, fifty made of steel, ...
1k views

### Agent 001's faulty delivery box

There are 10 agents living at their respective homes. They used a secret mailing system that sends one box at a time, but it is now spoilt. Now, it functions as follows: It counts the number of ...
7k views

### Salesman's claim for mechanical keypad lock - 5 buttons and 545 combinations!

A company makes mechanical keypad locks. The keypad is a set of five buttons arranged vertically. O O O O O The buttons are quite close together. Once a ...
3k views

### Do you have my spare key?

I have three bicycle locks, ones like this: Because they look so much alike I etched a unique number into each lock and the same number into the keys that unlock it. Unfortunately I seem to have lost ...
4k views

### 2 Guards, 3 Keys, 2 Locks

There are 2 guards: one only tells the truth, the other only lies. There are 2 locks and 3 keys: each lock can only have 1 key matched - the 3rd key doesn't do anything. You can only ask one ...
903 views

### How do you create a general locked-box problem?

In this question, I presented a puzzle involving seven locks, with a Grand Vizier and four slaves. But I would like to use this style of puzzle in some of my own secret-unlocking riddles, in order to ...
5k views

### The Puzzle of the Locked Box

Here's an old puzzle I found in a book once. A box has some number of locks on it. It is guarded by a Grand Vizier and four slaves, each of which hold keys to the locks. The Grand Vizier and ...
3k views

### Cracking a combination lock

Let's consider the humble mechanical combination lock. The basic type has $n$ dials, with $k$ digits on each dial, for a total of $k^n$ possible combinations. (Typical luggage locks or suitcase lock ...
489 views

### One Hundred Lockboxes, Variant

You have one hundred lockboxes, fifty made of steel, fifty made of wood, and a key for each box. A prankster breaks into your house and distributes the keys among the boxes. He places each key in a ...
1k views

### Five keys, five doors, seven guards, and only seven questions

Trapped in a dungeon, you have found five keys, labeled A, B, C, D, and E. You have five locked doors, one in front of the other, numbered 1, 2, 3, 4, and 5. To escape, you must open all five doors. ...
655 views

### $100 + n$ boxes for the king!

There are 100 friends and $n$ enemies in a room. In the next room, there are $100+n$ open boxes, and $100+n$ keys, one corresponding to each box. They are numbered for easy identification. A box can ...
1k views

Your progress is barred by a locked door, it is clearly controlled by a keypad. with 10 digits (0-9) plus a button for ENTER and a button for CANCEL. You know several things about this lock: This ...
1k views

### How to lock a shared artefact

This answer reminded me of a nice puzzle. I present it as a challenge for you. N thieves had stolen a magical artefact. They bought a lock with N keys and attached the artefact to a loop on a wall ...
337 views

### Identical-Looking Keys on a Ring

The number of keys on your key ring has been growing steadily over time, and now you already have 15 keys to keep track of! Unfortunately, you are unable to distinguish one key from another (apart ...
668 views

### Key and lock puzzle

The idea is to place all the yellow keys into the red locks. There is at least one guaranteed way to do this as the locks as created from the keys and then 'blurred' by adding extra gaps. Solutions ...
529 views

### Lock Optimisation

In a simple combination lock, a sequence of several digits is used as a password, with one wheel per digit. Generally, it is not possible to unlock the lock without knowing the password, but due to ...
876 views

### What's the door code? [closed]

You're running late to your first day on the job and after arriving at the meeting location, you see the orientation group just leaving without you through a door with a keypad. Panicked, you confirm ...