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Questions tagged [mathematics]

A puzzle strongly related to mathematical facts and objects, or whose solution needs serious mathematical arguments. General mathematics questions are off-topic but can be asked on Mathematics Stack Exchange.

22
votes
8answers
3k views

How to choose at least half of everything

Some number of gold, silver, and copper coins are scattered in $N$ chests. You may look into each chest and count each type of coin in them, and then select $M$ of the chests. Your goal is to have at ...
5
votes
2answers
11k views

Explain the Magic Numbers!

The following are tricks with numbers that you must explain how they work. Trick #1 Think of a number between 1 and 9 Multiply by 9 Add up all of the digits of this number Divide by 3 Subtract 2 ...
4
votes
3answers
904 views

The generous team puzzle

An intelligent team, ‘Smarty Pants’, applying to a competition completes an evaluation form early. Since they are feeling generous they randomly spread their assignment to all 10 teams applying from ...
5
votes
4answers
3k views

Are you Really Smart

The government has introduced a standardised test to identify dumb people and make them ineligible to apply for any government jobs, a pass in the exam is mandatory for applications in future. It ...
4
votes
3answers
1k views

Crack the Code #4

The park bench has been compromised as a safe location, so you decide to meet in the lobby of the nearest hotel. Again, the envelopes are traded securely, along with the money you are receiving for ...
5
votes
3answers
922 views

Crack the Code #3

You return to the bench after cracking the last code, and you notice an envelope on the ground directly under it. You put down your envelope with the previous code and pick up the new one, ...
9
votes
3answers
4k views

Is my sibling a sister [duplicate]

A male friend of yours wants to test if you are ok at Math and logical thinking, he says I am one of two children of my parents, and asks you to find out the probability that his sibling is a sister, ...
7
votes
4answers
2k views

Crack the Code #2

After the last bank encounter, you decide that you could make a living off of just cracking codes (as if robbing banks wasn't enough). A shady man meets you on a park bench and slides you an envelope, ...
35
votes
1answer
4k views

Ernie's automatic parking valet

I was helping Ernie out in his shed a while back, when his favorite screwdriver suddenly wore out. I offered to go and pick up a new one for him and Ernie thought it was a great opportunity to test-...
7
votes
6answers
2k views

Crack the Code #1

You are a bank robber, and you come across a safe. It has a combination lock of 4 digits. Careful research (and stealing papers at the bank) has given you multiple clues as to the code - just barely ...
0
votes
2answers
1k views

6 Attempts to Guess a Number Between 1-100 ~ Ranges

You get get 6 attempts to find the number between 1-100. Each "attempt" can either be a guess, or a range it could fall in, responses you could receive are "Correct!", "My number is lower.", "My ...
4
votes
2answers
986 views

Another 6 tries to guess a number between 1-100

Inspired by 6 Tries to Guess a Number Between 1-100, how many guesses would it take if you can ask if the outcome of an operation is equal, less than, or more than a number. In other words, Alice ...
-3
votes
3answers
471 views

The 3 Boxes Show! [duplicate]

A variation of the Monty Hall, where the values inside the choices are changed You were just out gambling and lost 5000 dollars! Of course being the gambler that you are, you pay 2500 to play this ...
2
votes
3answers
701 views

4 Doors and a Half Value prize

You're on a game show, playing for 1 million dollars! You've made it to the final round, there are 4 doors, you pick a random door (obviously, there's a 1/4 chance for all of them, let's say you ...
6
votes
1answer
965 views

Non-Negative integers in to three groups

The non-negative integers are divided into three groups as follows: A = {0,3,6,8,9...}, B = {1,4,7,11,14...}, c={2,5,10,13...} Explain ? I have no idea how to proceed.
14
votes
3answers
27k views

10 digit number where first n digits are divisible by n

What is the complete set of 10-digit numbers using each value ($0$-$9$) once, such that the first $n$ digits form a number divisible by $n$ (for $n \in \{1,2,...,10\}$)? E.g. if the number were $...
17
votes
3answers
2k views

Baggage Problem: $1.5$-meter-long sword onto a train

This is the problem I came across reading the book The Art and Craft of Problem Solving. When I read this question I wasn't able to figure out the solution and I saw the solution after a while, but ...
7
votes
2answers
2k views

Can you help the traveller

A traveller decided to explore a square desert. He started from a point on the edge to point B, travelling perpendicular to the edge of the desert he started on. On reaching B, he then decided to take ...
0
votes
3answers
892 views

Inheritance by cards [duplicate]

An old farmer passes away in his sleep. After a suitable period of mourning, his three sons find their father's will in his study. Amongst other, simpler bequests is the following: "I wish my small ...
4
votes
1answer
388 views

Housing crisis in heaven

Heaven has a countably infinite number of cloud for people to stay, each of which has a good soul living on it. A countably infinite number of infinite universes are extinguished by God (their beta ...
22
votes
4answers
3k views

Ernie's Clockwork Calculator

I was helping Ernie clean out his shed a while back when I saw a curious old machine. "Oh yeah, that!" said Ernie, when I pointed it out to him. "A few years back, before transistors were invented, I ...
3
votes
5answers
939 views

A belt, a stone and a wooden stick

We're on the middle of the most devastating war the Earth has gone through all its history. You and your brother in arms are in a plane that has been taken down while crossing the enemies lines. You ...
-6
votes
3answers
930 views

When Math Loses Meaning

In an alternative universe where the following is true: 1 # 1 = 3 2 # 2 = 7 1 # 2 = 5 What would ? be? 5 # 5 = ? YOU DO NOT NEED THE FOLLOWING TO FINISH. Extra equations:
6
votes
1answer
2k views

Generalization of Sum and Product puzzle

I heard from a clever person that for the problem mentioned before: I don't know the two numbers... but now I do a solution exists even if we change 100 to infinity. So the formulation would be ...
2
votes
2answers
849 views

100 sequential parking spaces

I just came up across this question in the mathematics stack exchange site and found it quite interesting: 100 autonomous robotic vehicles enter a warehouse in random order to park. Inside the ...
41
votes
17answers
8k views

A blanket for my baby snake

Mama snake wants to knit a blanket for little baby snake. She is not a dissipater and wants to make the blanket of a minimal size (area). But her baby snake is quite a lively baby and it always ...
1
vote
3answers
1k views

How many equal-sized polygons can be used to cover a soccer ball?

It seems that mostly either 26 or 32 equal-size polygons are used to cover a spherical soccer ball. What other configurations are possible?
118
votes
8answers
19k views

The Sheikh dies

The Sheikh dies, leaving behind three sons, 17 camels and the following order: His oldest son shall inherit one in two camels. His middle son shall inherit one in three camels. His youngest son shall ...
6
votes
3answers
2k views

Amount of hair on the head

Can you prove in two different ways that at this moment, some two persons on Earth have exactly the same number of hairs on the head?
4
votes
3answers
545 views

Flowers For a Patient

A woman wanted to visit her friend at the hospital. She had walked 500 meters from her front door before remembering that she wanted to pick up some flowers. She went back to the florist she had ...
6
votes
3answers
975 views

What is the next 3 numbers?

These numbers are created with a formula. Guess the formula for increment and calculate at least 3 next numbers of this sequence. increment is started with two given first $1$s. $1,$ $1,$ $2,$ ...
10
votes
1answer
2k views

20 coins on the table

Two boys , Brian and Gabe, decide to play a game. They lay 20 coins on the table. Each player took turns taking 1, 2, or 3 coins. Whoever took the last coin won. Brian thought if he went first, he'd ...
34
votes
3answers
5k views

Do better than chance

You and an opponent are playing the following game: Your opponent picks two different real numbers, however he wants to, and writes them on two cards, which he presents to you face down You choose ...
6
votes
2answers
2k views

Find the age of the ship and of the boiler

Some years back I was travelling by a cargo ship from New Zealand to Tahiti. I was curious to look around the ship one day and in the boiler room I asked a man how old the ship was. He smiled and ...
1
vote
3answers
460 views

Balance the Equation

Peter found an old piece of paper that an equation was written on it. Some of digits or operations are unreadable. Help Peter to complete the balanced equation. The equation is: $87+?3?12-4?2=?36-15??...
7
votes
1answer
399 views

Shortest time interval measurable via burning a rope

You are given some matches, a rope, and a pair of scissors. The rope burns irregularly and takes 60 minutes to burn end to end; however, we know that the burn rate at distance $x$ from the left end is ...
17
votes
1answer
3k views

The Laziest Surveyor

The new land surveyor and his boss arrived at the work site, a huge and perfectly flat parking lot. His boss said, "Your first assignment is going to be a tough one. You see this parking lot? It ...
12
votes
2answers
1k views

How to ship the new Slurm 7-pack efficiently

The six-pack is a thing of the past. Beverages of the future will use the seven-pack format. But how will the mighty spacemen of the future manage to ship the Slurm seven-pack efficiently in ...
6
votes
1answer
2k views

What is the number 4.9292683…?

Guess the next digits of this number. Where does it came from? 4.9292683
-3
votes
2answers
657 views

Infinite Troll Tolls

So, recently there was a very good riddle posted here: Paying the Troll toll. I would like to propose a similar question which might have been partially answered. But anyways... So, same basic idea ...
5
votes
3answers
858 views

Witches and Cats

This being Halloween, we will consider the problem of witches and their cats. The whole crowd is out tonight, naturally, and an interested observer, seeing them fly by (yes, witches' cats fly), ...
6
votes
2answers
532 views

Odd birthdate surprise

Last year, just before my birthday in February, I noticed something unsual. The last two digits of my birth year were the same as my age. How old was I then and was this an unusual occurrence?
13
votes
2answers
1k views

Horses and Tracks

I have seen this question before and I am intrigued that there might be various ways it can be solved. I do know of one way, but I am trying to find out if there is any other approach possible. The ...
149
votes
1answer
7k views

Chaos and Order: a visual puzzle in stained glass

I created a visual puzzle, which my wife then implemented as part of a stained-glass window. I've no idea if it is (a) obvious, (b) stupid or hopefully (c) extremely clever, and hence would love to ...
15
votes
2answers
782 views

Santa Claus flies to the South Pole

It is a little known fact that Santa Claus is in fact a vampire. Why else would he spend so much time near the North Pole and travel only at night? Yes, he fears daylight! His problem is that ...
-1
votes
1answer
477 views

Products of three primes that bound RSA numbers [closed]

Let $N$ be some number. Let $R$ and $S$ be semiprimes (ie, each is a product of two prime numbers) such that $R \le N \le S$. For quite a few numbers $N$ we can find $R, S$ such that $S-R$ is small;...
9
votes
2answers
1k views

Entangled physicists

The anti-intellectuals have won the war, and under the iron rule of emperor Nonotin anyone who has been deemed too smart for their own good has been brought to labour camps. But most of the science ...
-2
votes
1answer
700 views

Find the Different Sugar Cubes [duplicate]

There are 10 Boxes, each contains 1000 sugar cubes. The sugar cubes in 9 of the boxes are 10 grams each and the other box has 9 gram cubes. There is a digital scale that you can use it ONCE. (i.e. ...
3
votes
2answers
1k views

Unknown Consecutive Integers Puzzle

Two people, Anne and Bob, each hold an index card. A positive integer is printed on each index card, and these integers are consecutive (i.e. Anne's card may display 8 and Bob's card 7). Anne and Bob ...
22
votes
4answers
3k views

The Monty Hall problem

I'm amazed that, although well known, this problem hasn't been posted yet. Monty Hall You are on a game show. The host of the show shows you three doors. He tells you that behind one of them, is a ...