If n persons are sitting in a row, find the number of ways of selecting two persons, who are sitting adjacent to each other.

10 persons are sitting in a row. Find the number of ways of selecting two persons out of them who are sitting adjacent to each other

If 20 persons are sitting in a row, find the number of ways of selecting 3 persons out of them so that no two of the selected three are consecutive.

Ten persons are arranged in a row . The number of ways of selecting four persons so that no two persons sitting next to each other's are selected is

Ten persons are arranged in a row.The number of ways of selecting four persons so that no two persons sitting next to each other are selected is

14 persons are seated at a round table. Find the number of ways of selecting two persons out of them who are not seated adjacent to each other.

12persons are seated at a round table. Number of ways of selecting 2persons not adjacent to each other is