At a party, everyone shook hands with everybody else. there were 66 handshakes. how many people were at the party?

N - the number of people
Everyone shook hands with everybody else, so we must know how many ways there are to choose 2 people from the group of n people. Using combination:


There were 66 handshakes.


The number of people must be a positive number, so n=12.
There were 12 people at the party.

The right answer should be 6 but your not very specific