if n persons are seated in row the number of ways of selecting 3 so that no two r seated next to each other

Dear Shiwani,

Required Prob,  = select 3 persons from n  -  consider 2 persons sitting together as one so total persons are n-1 and now select 3 persons from n-1

                      =  nC₃ - (n-1)C_3*2

Note : (n-1)C_3*2 bcoz 2 persons can be arranged in two ways .

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