(i) A committee of 7 has to be formed from 9 boys and 4 girls.

Since exactly 3 girls are to be there in every committee, each committee must consist of (7 – 3) = 4 boys only.

Thus, in this case, required number of ways

(ii) Since at least 3 girls are to be there in every committee, the committee can consist of

(a) 3 girls and 4 boys or

(b) 4 girls and 3 boys

3 girls and 4 boys can be selected in ^{4}C_{3} x ^{9}C_{4} ways.

4 girls and 3 boys can be selected in ^{4}C_{4} x ^{9}C_{3} ways.

Therefore, in this case, required number of ways

(iii) Since atmost 3 girls are to be there in every committee, the committee can consist of

(a) 3 girls and 4 boys (b) 2 girls and 5 boys

(c) 1 girl and 6 boys (d) No girl and 7 boys