Five friends are having tennis tournament. Each friend play the other four friends once. How many matches will be played?

10 matches.

Player A, B, C, D, E

A-B, A-C, A-D, A-E

B-C, B-D, B-E

C-D, C-E

D-E

This is one match each per person.

A-B in first match. So if you go to B bracket. He can't play A again, so A is not included. And naturally he cannot play himself. B-B cannot happen. So you start as B-C.