I have found the answer to this RMO problem that baffled everyone

Suppose 32 objects are placed along a circle at equal distances. In how many ways can 3 objects be chosen from among them so that no two of three chosen objects are adjacent nor diametrically opposite?


The answer is 3616.

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3 solutions

Mark Hennings
Dec 11, 2015

There are 32 C 3 = 4960 {}^{32}C_3= 4960 triples altogether.

Of these, 16 × 30 = 480 16 \times 30 = 480 contain diametrically opposite points ( 16 16 choices for the diametrically opposite points, and 30 30 choices for the other point).

A total of 32 × 28 + 32 = 32 × 29 = 928 32\times28 + 32 = 32 \times 29 = 928 triples contain adjacent points ( 32 × 28 32 \times 28 of these triples have just two consecutive points, while the other 32 32 have three consecutive points).

A total of 16 × 4 = 64 16\times4 =64 triples contain adjacent points and diametrically opposite points.

Thus 4960 480 928 + 64 = 3616 4960 - 480 - 928 + 64 = 3616 triples contain neither adjacent points nor diametrically opposite points.

Shubhendra Singh
Dec 10, 2015

Arrange all things in a straight line a 1 , a 2 . . . . a 32 a_{1},a_{2}...….a_{32}

See the following arrangement

P a Q b R c S P \boxed{a} Q \boxed{b} R \boxed{c} S

Here a, b, c are any three things P, Q, R, S are no of things left in their middle and aside.

Here P + Q + R + S = 29 P+Q+R+S =29 where P , S 0 & Q , R 1 P, S \geq 0 \& Q, R \geq 1

So the no. of solutions of this equation are 27 + 4 1 C 3 = 4060 ^{27+4-1}C_{3}=4060 . Now here we had taken them in a line.

When arranged in a circle a 1 a_{1} and a 32 a_{32} can't be together so we will remove the cases in which both of them are together, there are 28 cases to be removed.

So now we get 4060-28 = 4032 cases

Now we have to remove the cases of diametrically opposite things.

Here two things a p ; a q a_{p}; a_{q} p>q are diametrically opposite if p q = 16 p-q=16 we will get 16 pairs with 26 arrangements corresponding to every pair so total cases are 26×16=416

Finally the answer comes out to be 4032-416=3616

Rwit Panda
Dec 29, 2015

Total cases are 32 c 3 = 4960 32c3 = 4960

Cases to be subtracted:

1) All 3 adjacent = 32

2) 2 adjacent and third one not adjacent to both= 32*28 = 896

3) 2 diametrically opposite and third one not adjacent to both = 16*26 = 416

So, answer is 4960 32 896 416 = 3616. 4960 - 32 - 896 - 416 = 3616.

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