I. Matches of first player with other 5 players. II. Matches of second player with 4 players other than the first player. III. Matches of third player with 3 players other than the first player and second player. IV. Matches of fourth player with 2 players other than the first player, second player and third player. V. Matches of fifth player with 1 player other than the first player, second player, third player and fourth player. So total matches will be 5+4+3+2+1=15