Given number of boys = 9

Number of girls = 4

Now, A committee of 7 has to be formed from 9 boys and 4 girls.

1. If in committee consist of exactly 3 girls:

     ⁴C₃ * ⁹C₄

= {4! / (3! * 1!)} * {9! / (4! * 5!)}

= {(4*3!) /3!} * {(9*8*7*6*5!) / (4! * 5!)}

= 4 * {(9*8*7*6*) / 4!}

= {4 * (9*8*7*6*)} / (4*3*2*1)

= 9*8*7

= 504

2. If committee consists of atleast 3 girls:

   ⁴C₃ * ⁹C₄ + ⁴C₄ * ⁹C₃

   = [{4! / (3! * 1!)} * {9! / (4! * 5!)}] + 9C3

   = [{(4*3!) /3!} * {(9*8*7*6*5!) / (4! * 5!)}] + 9! /(3! * 6!)

   = [4 * {(9*8*7*6*) / 4!}] + (9*8*7*6!)/(3! * 6!)

   = [{4 * (9*8*7*6*)} / (4*3*2*1)] + (9*8*7)/3!

   = (9*8*7) + (9*8*7)/(3*2*1)

   = 504 + (504/6) 

   = 504 + 84

   = 588

3. If committee consists of atmost 3 girls:

  = ⁴C₀ * ⁹C₇ + ⁴C₁ * ⁹C₆ + ⁴C₂ * ⁹C₅ + ⁴C₃ * ⁹C₄

  = 36 + 336 + 756 + 504

  = 1632