Team one must play against 12 different temas => 12 days
Team two, already played with team one, then only has to play 11 more days
Team three, already played with teams one and two, then there are 10 dates left.
So on, the result is 12 + 11 + 10 + 9 + 8 + 7 + 6 + 5 + 4 + 3 + 2 + 1 = 78 days.
You can also find that the number of different games is 13*12 / 2! = 78
Also, you can do Combinations of 13 taken 2 at a time: 13C2 = 13! / [(2!)(11!)] = 13*12*11! /(2!*11!)= 13*12/2 = 78.
