First we find the number of teams of 4 students without regard to gender. 20 students Choose 4 = 20C4 =Then we find the number of teams with all 4 boys to subtract from that number. 8 boys Choose 4 = 8C4 = And we find the number of teams with all 4 girls to subtract from that number, too. 12 girls Choose 4 = 12C4 = Answer: 4845 - 70 - 495 = 4280. Edwin