a group of five children has to be chosen from a form of eight boys and twelve girls.in how many ways can the selection be made if the group must contain at least two boys and two girls?
That's either 3 girls and 2 boys or 2 girls and 3 boys.
Case 1: 3 girls and 2 boys
Choose the 3 girls 12C3 = 220 ways and the 2 boys 8C2 = 28 ways.
That's 220*28 = 6160 ways.
Case 2: 2 girls and 2 boys
Choose the 2 girls 12C2 = 66 ways and the 3 boys 8C3 = 56 ways.
That's 66*56 = 3696 ways.
Grand total 6160+3696 = 9856 ways.
Edwin