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.
211
questions
9
votes
3answers
577 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
138 views
3 bags of coins, find which bag contains fake coins, doing only 1 weighing
Probably a very easy puzzle for you guys; my point is more a mathematical one.
FYI, it was shown in an episode of Columbo.
You have 3 bags of coins. Each bag contains 50 identical coins. Two bags ...
0
votes
0answers
79 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
422 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
691 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
745 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
548 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
544 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
586 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
216 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
1answer
896 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
146 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
121 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
172 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
181 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
751 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
217 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
545 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
614 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
74 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
60 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
84 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
456 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
877 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
383 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
204 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
645 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
329 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
230 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 ...
20
votes
4answers
839 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.
8
votes
5answers
656 views
The Legend of Four
As far as I know, all numbers have a root of 4. What I mean by this is as follows:
Starting with any number, for example 384, I take the number of letters in that number. Then I repeat this process ...
13
votes
2answers
724 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 $x$?
2
votes
2answers
369 views
Can the sum, difference and product of 2 numbers be perfect squares? [closed]
If we take 2 numbers $x$ and $y$ such that $x>y>0$ and , can $x + y$, $x - y$ and $xy$ all be perfect squares?
3
votes
2answers
89 views
Can you have the harmonic, geometric, arithmetic and quadratic mean of 2 numbers all being integers? [closed]
I created this problem looking at the hm-gm-am-qm inequality (hm = $\frac{2xy}{x + y}$, gm = $\sqrt{xy}$, am = $\frac{x + y}{2}$, qm = $\sqrt{\frac{x^2 + y^2}{2}}$ and hm $\le$ gm $\le$ am $\le$ qm).
...
11
votes
4answers
5k 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 ...
4
votes
8answers
533 views
Finni's tricky game
Finni’s game:
Person A thinks of a number (1 to 10). This number is called n.
Person B says a number (1 to 10). This number is called x.
Person A tells the absolute difference of n and x. This ...
3
votes
1answer
160 views
Three-digit multiplication puzzle, part III: Return of the Hex
Followup to: Three-digit multiplication puzzle and Three-digit multiplication puzzle, part II: ever heard of senary?
Place different three-digit hexadecimal numbers (000-FFF) on each of the seven ...
3
votes
2answers
158 views
Three-digit multiplication puzzle, part II: ever heard of senary?
Followup to: Three-digit multiplication puzzle
I'd never heard of "senary" either, until creating this series of three puzzles. It seems that "senary" is the word for base-six notation. Who knew? ...
4
votes
1answer
193 views
Three-digit multiplication puzzle
Place different three-digit numbers (000-999) on each of the seven nodes of the following diagram:
There are six lines in the diagram, on each of which are three of the nodes. On each of those lines, ...
7
votes
1answer
517 views
Fourteen numbers around a circle
Place the numbers 1 to 14 around this circle so that both the sum and (absolute) difference of any two neighboring numbers is a prime.
6
votes
2answers
335 views
The Magic Letter H
Place seven different positive integers on the empty disks of the H figure below so that the product of the three numbers in any straight black line is always the same. Now place seven other numbers ...
14
votes
1answer
751 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 ...
