If there are 72 games in a series and each team plays every other team twice, once at home and once away, how many teams are there?
each of n teams plays n-1 teams twice
... but A playing B is the same game as B playing A
n * (n-1) * 2 * 1/2 = 72 ... n^2 - n = 72
n^2 - n - 72 = 0 ... (n - 9)(n + 8) = 0