High school reunion

Question

21 people came to celebrate their ten-year high school reunion. Each
person shook hands once with all the other persons. How many handshakes were
exchanged at the reunion?

Answer 1

Put then in a line.

Person 1 shakes 20 hands

Person 2 shakes 19 hands

Person 3 shakes 18 hands

 

and so on

 

Can you add them up?