Welcome to MindCipher, a social repository of the world's greatest brain teasers, logic puzzles and mental challenges.
5 11 The Stealing Mailman
This is a fairly simple problem that can cause some people trouble. Kevin and Sophia want to mail high confidential letters between each other. They know that the mailman is crooked and opens the le ...
logic cryptography Submitted by Ravi
Doable
6.5
6 3 Clever Commuter
Every day, Fred takes the train to travel from his work back to Alkmaar, his place of residence. Usually, he arrives at the station of Alkmaar at six o’clock, and exactly at that moment he is picked ...
Submitted by Valentin
Hard
7.5
4 4 Left Hands Only
Assume there are approximately 6,000,000,000 (6 billion) people on Earth. What would you estimate to be the result if you multiply together the number of fingers on every person’s left-hands? For t ...
Submitted by Aakash
Hard
6.8
2 6 The Easy Sum of 1..100?
Quick,without using a calculator, what is the sum of all numbers between 1 and 100 inclusive? Or 1..10 for that matter..
Submitted by Valentin
Easy
2.5
3 1 What number is next?
What number comes next in this sequence? 4, 20, 56, 120, 220, ?
Submitted by Aakash
Doable
4.3
3 4 Power of 2 (Microsoft Interview)
Given any number n, using one line of C (using no external libraries or functions) code, determine if n is a power of 2. Return 1 if n is a power of 2 and 0 if not.
bitwise powers Submitted by Aakash
Doable
6.0
5 A Pharmacist's Nightmare
You have 5 jars of pills. Each pill weighs 10 gram, except for contaminated pills contained in one jar, where each pill weighs 9 gm. Given a scale, how could you tell which jar had the contaminated p ...
Submitted by Ravi
Hard
7.0
0 4 Cutting Numbers
When is Seven half of Twelve?
lateral thinking Submitted by Ravi
Easy
3.0
0 4 Plane In The Wind
An airplane flies in a straight line from airport A to airport B and back. It has constant engine speed and no wind. Will its total flight time increase, decrease or stay the same if throughout bot ...
Submitted by Valentin
Doable
4.0
2 3 Lockers
A high school has a strange principal. On the first day, he has his students perform an odd opening day ceremony: There are one thousand lockers and one thousand students in the school. The princip ...
Submitted by Aakash
Hard
7.1
2 3 The Way Engineers Think
A priest, a doctor, and an engineer were waiting one morning for a particularly slow group of golfers. Engineer: What’s with these guys? We must have been waiting for 15 minutes! Doctor: I don’t ...
Submitted by Aakash
Doable
6.0
4 3 Up and Down
Explain the following scenario: A man lives on the twelfth floor of an apartment building. Every morning he takes the elevator down to the lobby and leaves the building. In the evening, he gets int ...
Submitted by Valentin
Hard
7.3
28 45 12 Identical Balls
You have 12 identical balls. One of them is slightly different in weight than the rest. You have three weighs of a balance scale to determine which ball is different and whether it is heavier or li ...
logic brain teaser Submitted by Valentin
Doable
6.2
1 3 Classic Knights & Knaves
Raymond Smullyan has made a career of writing logic problems. His most famous have to do with a mythical island of Knights and Knaves. Knights always tell the truth, and Knaves always lie. As you a ...
Submitted by Ravi
Doable
5.5
7 10 Cooking Time
A cook must put a souffle exactly 9 minutes at the oven, but he only have a 7 minutes hourglass and a 4 minutes hourglass. How can he measure 9 minutes with those clocks?
logic Submitted by Ravi
Doable
5.7