These questions keep reappearing in Aptitude segment of any competitive exam. XYZ number of people were invited in a party, how many handshakes were done etc. etc. These two videos should clear the concept.
Concept #1 : Handshakes after Party + Readymade Formula
At the end of a party, every person shook hands with other and total 28 handshakes were exchanged. Find out how many people attended this party? (APFC 2004)
Answer and Explanation in this video: http://www.youtube.com/watch?v=5gxxBZkv-hg
Concept #2: Inviting one or more people to a Party
Amit has 7 friends, whom he wishes to invite to a party. Out of his 7 friends, 1 or more may accept the invitation.
In How many ways can Amit’s 7 friends attend the party?
Answer and Explanation in this video: http://www.youtube.com/watch?v=AqeHP3GbyVg
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