n (> 3) persons are sitting in a row. Two of them are selected. Write the probability that they are together.
Let E denote the event that the selected persons are sitting together.
As 2 persons can be selected out of n in nC2 ways
Out of n persons we can select two persons sitting together in (n-1) ways.
Because we have to select only one person next person is going to be automatically selected.
We can’t select last person because no one is sitting next to him.
∴ 1 person out of n-1 persons can be selected in (n-1) ways.
∴ P(E) =
Thus, P(E) =