Suppose objects are placed along a circle at equal distances. In how many ways can objects be chosen from among them so that no two of the three chosen objects are adjacent nor diametrically opposite?
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One can choose 3 objects out of 32 objects in ( 3 3 2 ) ways. Among these choices all would be together in 3 2 cases; exactly two will be together in 3 2 × 2 8 cases. Thus three objects can be chosen such that no two adjacent in ( 3 3 2 ) − 3 2 − ( 3 2 × 2 8 ) ways. Among these, furthrer, two objects will be diametrically opposite in 1 6 ways and the third would be on either semicircle in a non adjacent portion in 3 2 − 6 = 2 6 ways. Thus required number is ( 3 3 2 ) − 3 2 − ( 3 2 × 2 8 ) − ( 1 6 × 2 6 ) = 3 6 1 6