Question

In a group of 10 people, each person shakes hands with everyone else once. How many handshakes are there?

Answer 1

@freckles

Answer 2

let's reduce things a bit and say there are only 4 people

call them A, B, C, D

now make a 4x4 table like this

Answer 3

person A can't shake hands with himself, same for B, C, & D

so scratch out the main diagonal