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

7 votes

Numbers that are averages of their digits

There are For example,
Will.Octagon.Gibson's user avatar
13 votes

Interesting irrational number

Previous answers seem to say that there are two solutions: $$x=\frac{1 \pm \sqrt{5}}{2}$$ but for $$x=\frac{1 - \sqrt{5}}{2}$$ the base 10 representations of $x$ and $x^2$ are: $$-.618\dots$$ and $$.3 …
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0 votes

The infinite bag of billiards balls

Solution to Variation 2: So at 12:00 this ball will be labeled: All balls are relabeled in a similar fashion so at 12:00 the bag contains balls labeled:
Will.Octagon.Gibson's user avatar
6 votes
1 answer
793 views

Find the numbers (can’t use digits other than 1)

Can you find two numbers composed only of ones which give the same result by addition and multiplication? Of course 1 and 11 are very near, but they will not quite do, because added they make 12, and …
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3 votes
3 answers
469 views

The infinite bag of billiards balls

The following trio of puzzles comes from http://skepticsplay.blogspot.com/ My question is: What is the solution to variation 2? The original author of these puzzles described them as being abstract. …
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4 votes

Seven birds in search of food

There are 7 birds so there are Since 3 birds fly every day, One possible schedule for the birds (A,B,C,...,G) ensuring every pair of birds flies exactly once is:
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2 votes
1 answer
236 views

Partition the numbers so that the three group sums are equal

For which values of the positive integer $n$ is it possible to divide the first $3n$ positive integers into three groups each of which has the same sum? This puzzle is from a UK Junior Mathematical O …
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11 votes
3 answers
2k views

How far on average does the grasshopper need to travel to get home after its drunken jumping...

A grasshopper is jumping on a number line and starts at its home at zero (i.e., the “origin”). Its first jump will be length 1/2, its second jump will be length 1/4, its third jump will be length 1/8 …
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16 votes
1 answer
1k views

Can we find four identically colored numbers?

Take the positive integers {1, 2, 3, ...} and color them red, green or blue. Is it true that no matter what coloring is chosen, we can always find three distinct numbers x, y and z so that x, y, z an …
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2 votes

Prove that it is always possible for them to join the line so that the number of men in fron...

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10 votes
3 answers
1k views

Fill 4 circles with numbers so that all circle sums are equal

Here is a diagram created by overlapping four circles. The overlapping circles create eight regions. Put a number in each of the eight regions, using each of the numbers 1 to 8 exactly once, so that t …
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25 votes
11 answers
3k views

Prove that it is always possible for them to join the line so that the number of men in fron...

From the 2011 South African Junior Olympiad: Several people line up in single file. A solitary latecomer wishes to join the queue. Prove that it is always possible for them to join the line somewhere …
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11 votes
4 answers
3k views

Is it true that if he plays long enough there will be a series of consecutive days during wh...

Grandmaster Lev Alburt plays at least one game of chess a day to keep in shape and not more than 10 games a week to avoid tiring himself out. Is it true that if he plays long enough there will be a s …
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20 votes
1 answer
2k views

Reducing π to zero (again)

You are given the first 20 digits of π: 31415926535897932384. In each move, you can select a contiguous group of 5 digits and increase/decrease them all by the same integer, provided that each resulti …
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3 votes
1 answer
397 views

Can you arrange the positive integers so that certain numbers are relatively prime?

Can you arrange the positive integers $(1, \ 2, \ 3, \ \cdots)$ into a sequence $$T_1, \ T_2, \ T_3, \ \cdots$$ such that the sequence contains every positive integer exactly once AND if $i, \ j, \ k$ …
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