Since the ratio of girls and boys must be the same in each team, and all the competitors must be involved, it means that each team's ratio of boys : girls must be equal to the total ratio of Boys : Girls = 36 :27= 4 / 3.
Now 4 / 3, the most simplified version of 36 / 27, happens by dividing the numerator & demoninator by 9 (the greatest number it can divide by). So the greatest number will be 9 teams, each of 4 Boys & 3 Girls
