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

See Solution: How many people in party?

lets say there are n persons

first person shakes hand with everyone else: n-1 times(n-1 persons)

second person shakes hand with everyone else(not with 1st as its already done): n-2 times

3rd person shakes hands with remaining persons: n-3

first person shakes hand with everyone else: n-1 times(n-1 persons)

second person shakes hand with everyone else(not with 1st as its already done): n-2 times

3rd person shakes hands with remaining persons: n-3

So total handshakes will be = (n-1) + (n-2) + (n-3) +…… 0;

= (n-1)*(n-1+1)/2 = (n-1)*n/2 = 66

= n^2 -n = 132

=(n-12)(n+11) = 0;

= n = 12 OR n =-11

-11 is ruled out so the **answer is 12 persons**.