Question 578564
Each of the 10 teams plays 9 other teams, and 10*9 = 90. However, we are counting each pair twice (Team A, B and Team B, A should be counted as one game) so we divide by 2 to get 45. Alternatively, you can write the number of ways as 10C2.


This generalizes so for 20 teams, the number of games would be 20C2 = 190, and for n teams, nC2.