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.
224
questions
-2
votes
2answers
157 views
The last digit for 3^(2019) [closed]
Which would be the last digit for $3^{2019}$ ?
You can
And afterwards
16
votes
4answers
1k views
What's in my pocket?
Well, I can tell you Johnny has memory cards in his pocket.
Back Story
My brother, Johnny, is a tech nerd. He loves gadgets of all kinds. As a matter of fact, you can be sure at any one given time, ...
-1
votes
0answers
88 views
Maximize the Minimal Score [closed]
You are given 10 sets of papers with 10 questions each. Answer to each question is from 1 to 4.
You somehow get your hands on the answer sheets and able to memorize each answer (Answer sheets only ...
9
votes
3answers
419 views
Number Guessing (Part 1)
I thought up two positive integers with product less than $500$. I told their product to Penny, and their sum to Sandy, and told both of them the constraints and they are both perfect logicians.
...
5
votes
1answer
2k views
How to programmatically solve math puzzle
I have this puzzle and I want to solve by code. I wrote simple code trying to brute force but it fail in one condition
https://dotnetfiddle.net/cJyu3w
Anyone know of a link or source how to solve ...
19
votes
1answer
993 views
Four mathematicians and their ages
Four mathematicians, none yet a centenarian, meet for coffee. The graph-theorist among them noticed that the common divisor graph of their ages (that is, the graph whose vertices are their ages, two ...
4
votes
1answer
132 views
Playing Collatz with graphs
a) Given a set of positive integers, its common divisor graph (CD-graph) is the graph whose vertices are the integers, two of which are joined by an edge if (and only if) they have a common divisor ...
9
votes
5answers
2k views
Nice no-computers way to find limerick primes?
A limerick number is a 5-digit number whose digits are in the form of a limerick rhyme scheme: $aabba$. How many limerick primes (limerick numbers which are also prime) are there?
The answer is
...
11
votes
4answers
1k views
Professor Halfbrain and the number cycles
Yesterday afternoon I met professor Halfbrain at an art gallery.
The professor looked tired and exhausted.
He told me that he had spent many working days and many sleepless nights
with lengthy ...
-6
votes
1answer
205 views
Puzzling set of numbers [closed]
Let's have the following numbers $23, 40, 42, 44, \sqrt{43},\ 128i, 130, \sqrt{172}\ $. What is the relationship between these numbers, taken four at a time?
There are only two combinations when you ...
15
votes
6answers
637 views
A special triple of factors
Using each of the digits 1,2,3,4,5,6,7,8,9 exactly once, create three 3-digit numbers such that their product is a maximum.
5
votes
1answer
169 views
Generating special numbers [closed]
Here are three numbers that are related to one another: 9841, 8591, 4800
How are these numbers related and how are they generated?
11
votes
4answers
907 views
Making the whole set into primes
Let's say you start with a set of sequential integers starting from 2, so: $ 2, 3, 4, 5, \dots, N $ for some $ N > 2. $
The goal is to use identical basic arithmetic operations ($ +, -, \times, \...
6
votes
3answers
194 views
Find the value of $\bigstar$: Puzzle 10 - Uncertainty
This puzzle replaces all numbers with other symbols.
Your job, as the title suggests, is to find what value fits in the place of $\bigstar$. To get the basic idea, I recommend you solve Puzzle 1 ...
8
votes
3answers
634 views
Discover the six-character password!
You are given several pieces of paper which are as follows:
(Unfortunately, a textual rendering is very difficult with this puzzle, so if someone can offer a suggestion on how to do it, that would be ...
0
votes
0answers
81 views
Hitting a car with a bullet [duplicate]
1) There is a road on which a car starts with an integral speed towards the left or the right, starting from an integral point (take the road to be the x-axis)
2) The speed and the point from which ...
7
votes
2answers
435 views
Smallest prime number which when spelt out contains the letters P, R, I, M, E
So inspired by recent slew of questions based on prime numbers.
What is the smallest prime number when written out (Using the Western numbering system and English) would you encounter the letters P, ...
17
votes
4answers
3k views
Smallest PRIME containing the first 11 primes as sub-strings
In Smallest number containing the first 11 primes as sub-strings, @Alconja successfully found the smallest number which contains the first eleven primes (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31) as ...
10
votes
4answers
715 views
Sharing cake among 9 or fewer people
You are expecting guests to your birthday party. You know that there will be at most 8 guests, but you don't know how many will actually come. What is the smallest number of pieces you should divide ...
9
votes
3answers
749 views
The Royal Mint of Alphagonia
In the Kingdom of Alphagonia nothing can be bought for less than 30 alphas, the local currency.
1) What three denominations of coins should the kingdom mint so that as many as possible of the (...
8
votes
1answer
559 views
Winning the Lottery
Bob: I hear you won the lottery.
Alice: So I did!
Bob: What six numbers did you win it with?
Alice: Can't remember. All I recall is that they were all different, and none greater than 28.
Bob: ...
10
votes
2answers
560 views
National Graph Lottery
In UK's National Lottery players choose 6 different whole numbers in the range 1 to 59, and win a large prize if all six match with the day's draw.
Each choice of six numbers by a player gives rise ...
10
votes
2answers
594 views
A Magic Flying Saucer
Place 19 different positive integers on the vertices of this graph so that the 13 products of three numbers in a straight line are all equal. Do so in such a way that the product is as small as ...
11
votes
4answers
2k views
A Magic Diamond
Place 15 different positive integers on the vertices of this graph so that the ten products of three numbers in a straight line are all equal.
7
votes
0answers
239 views
What is a Freecell Word™?
This is in the spirit of the What is a Word/Phrase™ series started by JLee with a special brand of Phrase™ and Word™ puzzles.
If a word conforms to a special rule, I call it a Freecell Word™.
Use the ...
16
votes
3answers
1k views
Box of tablets, whole or broken: solution required
This is a puzzle that I thought up whilst taking a course of meds. I currently haven’t solved it, and would be curious to know if anyone has a solution for it. Here goes:
Scenario:
John has a box of ...
6
votes
2answers
147 views
Positioning cards labeled with numbers from 0-9
Ann and Bob play a game. On a table there are 10 cards which are labeled with number from 0 to 9 each. Bob is allowed to change the position of the cards with a sequence of his preference. When he is ...
5
votes
2answers
128 views
A pile of chips involving powers of 2
Ann and Bob play alternately on a pile of chips. On each play, any number of chips, which is a power of 2 (including 1=$2^0$), can be removed from the pile.
Obviously the number of chips to be removed ...
6
votes
1answer
175 views
A pile of chips involving primes
Ann and Bob play alternately on a pile of chips. On each play, either 1, 2 or 3 chips can be removed except if the number of chips is a prime number. In that case either 1, 2, 3, 4 or 5 chips can be ...
2
votes
2answers
191 views
How to find number of times sequence element $1$ is approached and from where?
Consider a sequence $1,-1,-1,-1,-1,-1,...,-1$. Start at the first element and move down the sequence according to the following rules:
1) If you jump from a $-1$ to another $-1$, turn the latter into ...
8
votes
2answers
755 views
IX-NAY on the IX-SAY
Will this sequence ever have a 6 in it?
9, 1, 1, 1, 10, 3, 1, 1, 10, 5, 1, 1, 10, 1, 5, 2, 1, 1, 10, 1, 1, 1, 5, 4, 1, 1, 10, 3, 1, 1, 5, 1, 1, 1, 5, 2, 1, 1, 10, 5, 1, 1, 5, 3, 1, 1, 5, 4, 1, ...
...
6
votes
1answer
220 views
I'm a Proper Divisor
Ann and Bob are playing a number game. Ann starts with the number 60. Then she subtracts a proper divisor of 60 from it. Bob then takes the number Ann made and subtracts one of its proper divisors ...
9
votes
4answers
547 views
The Football Squad
The sixteen players of a football squad, wearing shirts numbered 1 to 16, have arrived in town for a tournament. At their hotel, they are assigned 16 rooms consecutively numbered. Moreover, each of ...
12
votes
2answers
621 views
The Puzzling Reverse and Add Sequence
The sequence of numbers 0, 2, 4, 6, 8, 10, 12, 14, 16, 18, 11, 22,... (A056964 in the OEIS), in which the nth term equals n+reversal of digits of n, poses a number of intriguing puzzles. Here just ...
0
votes
0answers
78 views
Invert the numbers [duplicate]
I have an array of numbers containing 0 and 1 only and you are given a a constant C . You have to invert all the 0's to 1's by taking exactly C number of elements . What is the max number of 1's we ...
1
vote
1answer
208 views
Frobenius coin problem variation
Suppose you are give $n$ currency notes from $k$ to $k+n$
i.e $k, k+1,k+2.....k+n$
Where $k,n>0$
It's asked the total number of denomination of money that can't be formed using any number of ...
1
vote
0answers
61 views
How many ordered pairs (a,b) satisfy a^2=b^3+1, where a and b are integers? [closed]
(A)2
(B)3
(C)4
(D)5
I got 2 pairs (0,-1), and (3,2), but the correct answer is 5.
Can somebody help?
Thanks in advance!
-1
votes
2answers
109 views
If $a$ and $b$ are two odd distinct prime number and if $a>b$, then $a^2-b^2$ can never be divided by [closed]
(A) 13
(B) 11
(C) 17
(D) None of the above
3
votes
2answers
464 views
Let M and N be single-digit integers. If the product 2M5 x 13N is divisible by 36, how many ordered pairs (M,N) are possible? [closed]
Let M and N be single-digit integers. If the product 2M5 x 13N is divisible by 36, how many ordered pairs (M,N) are possible?
-- source
I tried it by reducing 36 into its positive factors (1,2,3,...
4
votes
2answers
1k views
A king was born in a year that was a perfect square, lived a perfect square number of years, and also died in a year that was a perfect square
In which year could he have been born?
(A) 1936
(B) 1764
(C) 1600
(D) 1444
The answer's (C). Why?
5
votes
4answers
882 views
Why did this prime-sequence puzzle not work?
While attacking a recent puzzle (whose solution ended up being entirely different from what I was trying), I was inspired to create a number-sequence puzzle with a sequence $(p_n)$ of primes where the ...
8
votes
1answer
403 views
Number sequence puzzle; 2, 10, 44 (+2 hints)
Predict the next three members of the sequence below and explain what the relationship is.
2, 10, 44, 1012, 248,
This number sequence does not appear in the Online Encyclopedia of Integer ...
6
votes
1answer
139 views
Find the immediate square dancing neighbors, they dance together to perfect square
We live in a community of houses sequentially numbered from 1 to 100. We all love square dancing but only two immediate neighbors are joy to watch. If you concatenate their house numbers, it forms a ...
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?
5
votes
2answers
213 views
SafeCracker #2 - The Mission Continues
Thanks to an alert StackE user, we were able to get the first safe open.
Mission Details
This next safe is in a former employee's house. He is gone for now, so we have no time to spare.
We weren't ...
8
votes
3answers
652 views
Double or Take game
Double or Take is a two-player number game. Alice starts by selecting any positive integer. Bob's options are to:
subtract a positive perfect square
subtract a positive perfect cube, or
double the ...
6
votes
2answers
1k views
Father and Son and Grandsons
The sum of the ages of a father, his son, and his two grandsons is 181. The father´s age has a common divisor greater than 1 with the ages of each of his three descendants, but not two of the latter ...
6
votes
2answers
335 views
Swap — A Puzzle I Created
This puzzle is called Swap. Let's find out why!
Suppose you are given a random $\rm N\times N$ matrix (grid) with all the integers from $1$ to $\rm N^2$ each belonging in every grid square (a.k.a. ...
-7
votes
2answers
238 views
What number + 1 equals itself? [closed]
Answer in numerical form.
The answer is not an integer.
The answer is in string form.
The answer is not my love life...
7
votes
2answers
1k views
An Accountant Seeks the Help of a Mathematician
The accountant complaints to the mathematician:
“I lent money to five other faculty members and still haven’t been paid back. You are one of them; the other four owe me 12 dollars altogether, but ...
