A double matrix will be the best bet here.
Attachment:
File comment: Double Matrix of toys
nuri graphics 500x500 (5).png [ 27.14 KiB | Viewed 406 times ]
(# of white toys for boys) + (# of blue toys for boys) = total # of toys for boys
Since half of the 1000 toys are for boys, 500 toys are for boys. 100 of those toys for boys are white, so that leaves 400 toys for boys to be blue.
500 - 100 = 400
(# of blue toys for girls) + (# of blue toys for girls) = total # of blue toys
Since three-fourth of the toys are blue, 750 toys are blue. That leaves 350 blue toys to be for girls.
750 - 400 = 350