An algebra problem by Anish Roy

Algebra Level pending

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


The answer is 3616.

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1 solution

Anish Roy
Aug 18, 2017

One can choose 3 objects out of 32 objects in ( 32 3 ) 32 \choose 3 ways. Among these choices all would be together in 32 32 cases; exactly two will be together in 32 × 28 32 \times 28 cases. Thus three objects can be chosen such that no two adjacent in ( 32 3 ) 32 ( 32 × 28 ) {32 \choose 3}-32-(32 \times 28) ways. Among these, furthrer, two objects will be diametrically opposite in 16 16 ways and the third would be on either semicircle in a non adjacent portion in 32 6 = 26 32 - 6 = 26 ways. Thus required number is ( 32 3 ) 32 ( 32 × 28 ) ( 16 × 26 ) = 3616 {32 \choose 3}-32-(32 \times 28)-(16 \times 26)=\boxed {3616}

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