1. The problem statement, all variables and given/known data There're 8 people in the room and each person shakes hand with the other . 2. Relevant equations How many hand shakes are there ? 3. The attempt at a solution One person will shake hand with 7 other people and so on . As a result there should be 8 power of 7 but the result is different ? P/S : Can you explain what's the difference between combination and permutation ?