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Questions tagged [number-theory]

A mathematical puzzle whose solution is heavily based on the arithmetic properties of the integers. General number theory questions are off-topic but can be asked on Mathematics Stack Exchange.

78
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18answers
14k views

Nine gangsters and a gold bar

One night nine gangsters stole a gold bar. When the time came for dividing the bar, they faced a problem: two of the criminals put guns to each other's faces. Now it's up to fate whether one of them ...
61
votes
24answers
19k views

Make 0 0 0 0 = 8

Can you find a way to make: $0\ 0 \ 0 \ 0 = 8$ by adding any operations or symbols? You can use only these symbols: $+,\ -,\ *,\ !,\ /,\ \hat\, ,\ ()$. It is limited to this list, and ...
31
votes
1answer
2k views

Professor Halfbrain and the sum of the digits of all divisors

Yesterday I met professor Halfbrain in the city. The professor looked tired and somewhat exhausted. He told me that he had spent his nights and days with adding up digits of divisors of positive ...
30
votes
13answers
5k views

Teacher, teacher on the wall, Who's the dumbest of them all?

A maths teacher writes a very large number on the blackboard and asks her pupils (of whom there are $n$ in the room) about its factors. The first pupil says, "The number is divisible by 2." The ...
30
votes
1answer
2k views

A Cave in the Black Mountains

Across the Deadly River, among the Black Mountains, is a mystical cave. Anyone who enters the cave finds within it a single gold coin, which may be freely taken. Once you leave the cave, you can never ...
30
votes
1answer
728 views

Integers around a circle with consecutive pairs adding to a square

The integers 1 to 50 are placed around a circle in such a way that the sum of any two of them which are adjacent is a perfect square. Of these integers, the even numbers are then removed. Restore them....
28
votes
1answer
3k views

The largest Monday number

A Monday number is a positive integer $N$ with the following three properties: The decimal representation of $N$ does not contain the digit 0 The decimal representation of $N$ does not contain any ...
26
votes
2answers
2k views

Fun with numbers: solve A to E

A, B, C, D and E are five distinct non-negative integers no greater than 99. Can you figure out their values based on the following simple clues? Not all of the five numbers have the same parity A + ...
22
votes
10answers
14k views

Make 5 5 5 5 = 19 [closed]

Can you find a way to make: $5\ 5 \ 5 \ 5 = 19$ by adding any operations or symbols? You can use only these symbols: $+,\ -,\ *,\ !,\ /,\ \hat\, ,\ ()$. It is limited to this list, and ...
22
votes
1answer
734 views

A Tour Around a Triangle

Place the 18 even integers between 2 and 36 in the empty nodes of this triangular graph in such a way that if a path is drawn by coloring in red all the edges joining any two nodes whose numbers add ...
21
votes
2answers
866 views

The year 2016 is approaching

Do there exist integers $r,s,t\ge100$ such that the decimal representation of $(r+\sqrt{s})^t$ is of the form $~~~\ldots\ldots2015\,.\,2016\ldots\ldots$? (In other words: the four digits before the ...
21
votes
2answers
3k views

The LoL number game

Tim and Tom are fictional characters (cats, I believe) who like to play the LoL number game. In this game, Tim chooses two distinct integers A and B, both ≥ 2, and shows them to Tom. Tom's goal is to ...
21
votes
1answer
613 views

A table filled with greatest common divisors

Yesterday I met professor Halfbrain at an art gallery. The professor told me that recently he had been spending his time with constructing $n\times n$ tables for integers $n\ge3$: the entry at the ...
20
votes
5answers
858 views

Professor Halfbrain and the powers of 2016

Yesterday I met professor Halfbrain at the coffee house. The professor told me that he had been spending his time with computing powers of $2016$ and combining them with powers of $32$. The professor ...
19
votes
6answers
6k views

3x3 “Magic Square” of Prime Numbers

During the thinking and analysis of some mathematical problems, I came up with this puzzle: Just like any magic square, one has to fill in $9$ different numbers $P_1, P_2, \dots P_9$ to a $3 \times 3$...
19
votes
4answers
739 views

Consecutive integers around a circle

Find a block of positive consecutive integers that can be placed around a circle in some order so that any two adjacent numbers always have a common divisor greater than 1.
18
votes
3answers
2k views

Professor Halfbrain and numbers with many zeros

Yesterday I met professor Halfbrain at the opera. During the break, the professor told me that he has made the following amazing discovery: Professor Halfbrain's theorem: With a finite number of ...
18
votes
2answers
2k views

Math logic Triangle

Rules Arrange numbers from 1 to 15 to the white triangles. A op B = result Green triangle means 2 numbers of its side are adjacent numbers. (-) means find the difference (/) means divide the bigger ...
17
votes
2answers
2k views

We are two immediate neighbors who forged our own powers to form concatenated relationship. Who are we?

Our concatenated number is $ \overline{ABAC}, $ where $ A, B, C $ are all positive digits (1 - 9). Our relationship is $$ \overline{ABAC} = A^A + B^B + A^A + C^C $$ Who are we?
17
votes
3answers
5k views

Which two students spoke wrongly? [duplicate]

A teacher wrote a large number on the board and asked the students to tell about the divisors of the number one by one. The 1st student said, "The number is divisible by 2." The 2nd student said, "...
17
votes
2answers
6k views

A general solution to the decanting problem? (aka jug-pouring, water-pouring)

Take a look at these two questions: - A Set of Water Jug Challenges - Pouring problem Now I'm asking for a generalised solution to that problem. I define the problem as follows: You are required ...
17
votes
1answer
490 views

A partition of 1000 into nine parts

The sum of nine whole numbers is 1000. If those numbers are placed on the vertices of this graph, two of them will be joined by an edge if and only if they have a common divisor greater than 1 (i.e. ...
16
votes
5answers
2k views

Divisible by seventeen

Determine the smallest integer $n \geq 0$ for which the decimal digit sum of n is a multiple of 17 the decimal digit sum of $n+1$ is a multiple of 17. No computers! The puzzle has a nice direct ...
16
votes
2answers
2k views

Fermat's Last Theorem - or is it?

Is there a solution in distinct positive integers $a,b,c$ to the equation $$a^3+b^3=c^4$$? If so, construct one; if not, prove that it can't possibly exist. Don't be too put off by the appearance of ...
16
votes
2answers
898 views

An arithmetic progression with primes

Here is a math puzzle I thought of a while ago: Find the longest arithmetic progression that consists only of primes, such that the difference between two consecutive terms is the product of two ...
16
votes
4answers
879 views

Weighing in 2015 different ways

This question was inspired by something I realized while thinking about @Gamow's Weighing in 89 different ways. You have a two-pan balance and $2015$ weights, with masses $1,2,4,\ldots, 2^{2014}$. ...
16
votes
1answer
544 views

Find the number from 10 statements

You are given the following ten statements and are asked to determine a particular number. At least one of statements 7 and 8 is true. This either is the first true or the first false ...
15
votes
5answers
2k views

Optimal Money-Saving on the NYC Metro

You are on vacation in New York City. You didn't bring your car, and it's currently around $-50^\circ C$, so it's probably a good idea to take the NYC metro subway to move around. You need a metro ...
15
votes
2answers
1k views

Find out the rule, then solve it

First, Find out the rule from the example, Then solve the puzzle without computer. The answer must be unique (just 1 valid answer). Example Solve This
14
votes
4answers
3k views

Mother and Daughter

A mother (not yet a centenarian) and her daughter (who happens to share her mother's birthday) are both a prime number of years old. Moreover, in their lifetimes there have been at least a dozen other ...
14
votes
2answers
586 views

Largest odd factors summing to a square

I just found this awesome puzzle from the Tournament of the Towns (though I'm sure it's appeared other places too). The connection between odd factors and square is surprising, and the proof has a ...
13
votes
5answers
4k views

My five daughters

The sum of the ages of my five daughters is 43. The ages of any two of them have a common factor greater than 1. How old are my daughters?
13
votes
3answers
1k views

Erasmus additions

Professor Erasmus has spent the last weekend by busily adding up all kinds of numbers. He told me today about his experiences: "On saturday evening, I added up twenty consecutive powers of $2$, and ...
13
votes
2answers
718 views

Digit sums of successive integers

For a natural number $x$ both, the digit sum of $x$ and the digit sum of $x+1$ are multiples of $7$. What is the smallest possible $x$?
13
votes
2answers
837 views

Delete a digit then sum

Take a number $(x)$, then create the complete list of the numbers formed by deleting single digits from its base ten representation $(d_1,d_2,...,d_n)$. If the sum of those new numbers equals $x$ we ...
13
votes
1answer
741 views

My Social Security Card Number

I have forgotten my social security card number. All I remember is that it is the largest integer with the property that the block of any two of its digits that are adjacent is either a two-digit ...
13
votes
1answer
844 views

The damaged QR Code

Consider the following pixel puzzle which somehow looks like a damaged QR Code with clues on the left of every row and on the top of every column. These numbers represent the total amount of "black ...
12
votes
4answers
10k views

Why is every prime number (5 and higher) divisible by 3 when you square it and subtract 1? [closed]

I discovered this by accident, when trying to create a formula that generates prime numbers (an impossible task, I know). But, I find it very interesting that you take any prime number 5 and greater, ...
12
votes
5answers
2k views

Ages of mathematician's five children

Two mathematicians meet at their school reunion. A: Hey old friend, I heard you have 5 children. How old are they? B: The sum of their ages is a cube number, A: But, I still don't know their ...
12
votes
1answer
525 views

A magical ordering of the positive integers

Professor Erasmus told me this morning that he has constructed a magical ordering of the positive integers, which the professor modestly calls "The professor Erasmus ordering of the positive integers"....
11
votes
11answers
10k views

Make 6 5 4 3 = 81

Can you find a way to make: 6 5 4 3 = 81 by concatenation and/or adding any of (and only) these mathematical operators: + - × ! ÷ ^ standard parentheses () You cannot add other numbers to the ...
11
votes
2answers
963 views

What number follows up next? Part 2

I'm trying to figure out what number follows next in this sequence. Can you help me? 5, 21, 341, 5461, 1398101, 22369621
11
votes
4answers
4k views

Create the numbers 1 - 30 using the digits 2, 0, 1, 9 in this particular order!

Inspired by the last year's "2018 four 4s challenge", I thought it's time to welcome 2019 by a similar challenge. This time you have to use the digits 2, 0, 1, 9 in this particular order to create the ...
11
votes
3answers
659 views

Three positive integers

Find the smallest possible value of $ab+c$, where $a,b,c$ are positive integers with $a+bc=2016$. (No computers! The puzzle has a nice direct solution.)
11
votes
2answers
1k views

Professor Halfbrain and the odd perfect number

Two hours ago, I received a phone call from professor Halfbrain. The professor told me that he has detected an odd perfect number. The professor was very excited, since the existence of odd perfect ...
11
votes
3answers
1k views

Four Marathon Runners

Four marathon runners, each identified with a positive whole number, sit around a table. Each of them notices that their own number has a common divisor with the number of the runner sitting on his ...
11
votes
1answer
329 views

Professor Halfbrain and the prime numbers

Professor Halfbrain has spent the last few days (and sleepless nights) with analyzing integer numbers of the form $~N_x(n):=n^x+x~$. The professor computed and analyzed thousands and thousands of ...
11
votes
1answer
1k views

Deducing Two Numbers based on their Difference and Ratio

Yesterday I met the perfect logicians Divvy and Subtra. I told them: I have chosen two positive integers $x$ and $y$ with $2\le x\lt y\le 100$ and with $x$ a divisor of $y$. I will now whisper ...
10
votes
6answers
2k views

Maximize the number of factorials in your solution to 6 5 4 3 = 1

Using the same rules as Make 6 5 4 3 = 1, but maximize the number of factorials. You may not take the factorial of 1 or 2. This is harder than it looks. A great answer has six factorials, an ...
10
votes
7answers
2k views

Last Digit of Multiplications

I have 4 different 1 digit positive integer numbers $(a,b,c,d)$. than I apply this formula $random(a,b,c,d) × random(a,b,c,d) × ... × random(a,b,c,d)$ the last digit of the result is always one of ...