# 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.

2,395 questions
877 views

### Fill in the number. Not as straight as it looks

The square below is missing a number: You have four options: 32 4 16 22 Please explain your answer. HINT:
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### Create a 3x3 Magic Square that uses integers from -10 to -2

I've never used this before but i'm having trouble with this. Create a 3x3 Magic Square that uses integers from -10 to -2. By the way, this is a weird magic square since diagonals don't need to ...
7k views

### Salesman's claim for mechanical keypad lock - 5 buttons and 545 combinations!

A company makes mechanical keypad locks. The keypad is a set of five buttons arranged vertically. O O O O O The buttons are quite close together. Once a ...
2k views

### Coin removal problem

I have a slight variation of the coin flipping problem. There is a line of $n$ coins on the table; some of them are heads up and the rest are tails up, in no particular order. The object of the ...
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### 5 subjects and their marks

The following partially-filled table shows the marks scored by 5 students named A, B, C, D and E in five different subjects, namely P, C, M, B and S. The marks scored by all students in all subjects ...
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### Grandpa’s baseball cards [closed]

One day at a family reunion we found a bunch of baseball cards in Grandpa's basement. Grandpa said we could split them among all the grandkids. There were 5040 cards in all, so each of us got a lot of ...
121 views

### Residential society [closed]

In a residential society, there are 120 people who either consume only tea or only coffee. Of those 120 people, 25 are children (which includes male as well as female children), and 78 consume tea. ...
119 views

### Ranking And Average

Four athletes A, B, C and D are evaluated on four different parameters – S, A, I and T. Further it is known that: I. Each athlete was given a score, which was an integer between 4 and 10 (both ...
295 views

### It's almost a cube

What is the next number in this sequence? 7, 10, 12, 19, 20, 30, _____ Note: I added another number as at first the pattern fit this too well. Hint 1:
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### Voting System Puzzle

Answer Questions 79 to 82 on the basis of the information given below: The year is 2089. Beijing, London, New York, and Paris are in contention to host the 2096 Olympics. The eventual winner is ...
123 views

### Length of the Sequence

There's a lot of number sequence based questions on this site. So I'm going to mix it up a little bit. This sequence will repeat eventually. You need to find how many numbers are in the sequence ...
449 views

### Solve this puzzle. I created for my friend for FUN :)

My friend is very good at puzzles. so i thought of creating one by myself and try to see if he can solve. Finally I succeeded and he gave up. Interested people can try. Hint: use all four arithmetic ...
300 views

### Rullo solving strategy for 2-4

How to eliminate/nominate candidates without guessing? For the 3s it's quite easy because of the odd numbers but how to get further?! For thoose who don't know the game, here's a short introduction:...
171 views

### Master and Slave versus Bob: easy version

Continuation of: "Sub-puzzle: Master and Slave versus Bob the Violent but Honest Psychopath" Continued in: "Master and Slave versus Bob: hard version" Previously I described a game that is played ...
5k views

### 100 using only 5 number of digits

This is a problem from Martin Gardner. For each digit $1\leq d\leq 9$, make 100 using exactly five number of $i$s. Any operation is allowed, brackets as well. For $i=1,2$ I have a solution: 100=...
144 views

### Row of alternating coin

I stumbled upon a mathematical puzzle that goes like this: You have 2n coins that alternate between head and tail. For example, if n=3, then it looks like H T H T H T, where H = head and T = tail. To ...
267 views

### A CrossNumber puzzle with a twist

Here's an interesting puzzle which I discovered the solution to yesterday. The eleven grid entries in increasing order are $A, B, C, D, E, F, G, H, I, J, K$, and you are given the following hints. ...
234 views

### What is the value of X?

24 6 63 132 4 14 23 x 17 The answer is either : 21, 14, 3 or 2
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### Master and Slave versus Bob: hard version

Continuation of: "Sub-puzzle: Master and Slave versus Bob the Violent but Honest Psychopath" and "Master and Slave versus Bob: easy version" The hard version is identical to the easy version (see ...
294 views

### Sub-puzzle: Master and Slave versus Bob the Violent but Honest Psychopath

[I created this as a sub-puzzle to "Strategy to beat the Casino reversed", that has been sitting unsolved for months. You may wish to read that puzzle first, but this is self-contained. My answer to ...
150 views

### More Email Shenanigans

I don't know how these people know my email address, but after the (still unsolved) shenanigans of the Interesting email I received? I got... yet another email. This one was of a Google Drive document ...
260 views

### My brain froze trying to solve this [closed]

Try to figure out the missing numbers!
209 views

### Using 20, 17, 16, 8, 7 to make 9

You can use $+, -, \times,$ or $/$ (namely $\div$). Use all five numbers (20, 17, 16, 8, and 7) only once to make $9$. From what I've been told there are $4$ answers. The only one I've been able ...
130 views

### Some Really Confusing Math

I was talking to a co-worker this morning and he stated that he was able to make both of these statements true using only $1, 3, 5,$ or $7$. He then proceeded to tell me: $1 = 5$ $2 = 4 + 7$ $3 = 1$ ...
432 views

### Which polygon is the one?

There is a unit-radius circle and you must form a polygon all of whose vertices are located on the circle, such as below: What is the biggest possible value of the sum of squares of side lengths of ...
1k views

### Did Grandpa count correctly? Another Grandpa Mystery

Grandpa just returned from a long trip. He started telling me about his adventure in a wild place. " I saw animals", he said. "Three different kind" "Just for fun I counted their limbs" "...
5k views

### Google's 20th Anniversary: Make 20 using 6 0 0 6 1 3

Belated 20th Happy Birthday to Google (600613)! Challenge: Use 6, 0, 0, 6, 1, 3 to make the number 20. Rules: Use all numbers, in that exact order. Allowed operations: +, -, x, /, sqrt, (); no ...
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### Is this possible ( - ) + + - - - = 100 I can do many other way

Insert the numbers 1-9 inclusive each only once into the calculation to arrive at the answer 100. ( - ) + + - - - = 100 Is this possible?
202 views

### Can you solve the riddle

Can you solve the mathematics puzzle?
105 views

### Air Miles Challenge

This isn't my own creation, it's an exercise that my partner had to do during a work exercise and seemed interesting to share here. You've been given 86,400 Air Miles to explore the world and there'...
439 views

### Cross-number puzzle

ACROSS: A. a square C. a multiple of N Across E. (C Down) $\times$ (I Across) G. a square root of A Down I. a square root of D Down J. (J Across) $\times$ (K Across) = (F Down) $\times$ (M Down) ...
165 views

### A Crucial Delivery

Planet Earth has been at war with the nefarious aliens of Planet XYZ for a while. The front lines are on Planet XYZ, and you, being Earth's lead scientists, need to find a way to get vital information ...
134 views

### Perkin's age problem [closed]

Perkin and Poise's combined age is 22 which is 3 years more than the combined age of Perkin and Pootle which is 2 years more than Poise's and Pootle's age combined. The question is - In how many ...
176 views

### The athletes puzzle [closed]

Athletes arrange themselves into $x$ rows and $y$ columns. We pick the tallest of every column and name the shortest of them A. We pick the shortest of each row and name the tallest of them B. ...