The number of ways 3 girls can be chosen=10*9*8=720
the number of ways 1 boy could be chosen=15
number of ways a committee could be formed=720*15=10800
Why multiplied by 9 and then followed by 8 ?
because. it is like make 3 boxes each representing one girl. you can place a total of 10 girls in that box. then as one of the girl is placed in the first block, 9 are left which can be placed in the second block and again 8 in the third block.