At a party, there were 105 handshakes. If each person shook hands exactly once with every other person, how many people were at the party?

Answer 1

If there are N folks, then
n-1 + N-2 + n-3 +....1 handshakes
take 4 people:
3+2+1= 6 handshakes.

10 people is
9+8+7+6+5+4+3+2+1= 45

14= 13+12+11+ 10 + number above
= 13+12+11+10+45= 91
you want 105, so try
15= 14+ 91

Answer 2

So the answer is 15? But still I don't get how you solve it