One shake is equal to

Everybody in a room shakes hands with everybody else. If the total number of hand shakes is 66, then the total number of persons in the room is

Every body in a room shakes hands with every body else. The total number of hand shakes is 66. The total number of persons in the room is

In a conference of 8 persons, if each person shakes hand with the other one only, then the total number of shake hands shall be