I do not get why there is "
9 matches within each group", if "
everyone will get to fight in all possible combinations" - there is more than 9 combinations... unless I got something wrong
We have 6 teams: (1), (2), (3), (4), (5), and (6); all combinations for team (1) are: 1-2-3, 1-2-4, 1-2-5, 1-2-6, 1-3-4, 1-3-5, 1-3-6, 1-4-5, 1-4-6, 1-5-6, that is 10 combinations for team (1)
for team (2): 2-3-4, 2-3-5, 2-3-6, 2-4-5, 2-4-6, 2-5-6, that is additional 6 combinations, all remaining (involving team (1) are already done
for team (3): 3-4-5, 3-4-6, 3-5-6 that is additional 3 combinations,
for team (4): 4-5-6 that is additional 1 combination.
That makes 20 matches within each group.... each team fights 10 times.