The formula for shaking hands: 1/2X(X- 1)= number of times (x is how many people are there)! Because you shake hands with me and I shake hands with you twice, so divide by 2! There are also single-cycle balls that are all this formula! For the round robin tournament, the greeting card and telephone number are: X(X- 1)= times (x is how many people are there)! Because you call me and I call you have two different meanings, so don't divide by 2! Satisfied adoption!