Every boy sees as many yellow bow ties as red ones: it proves that he doesn't see his own, so boys see more than girls 1.

Every girl sees the yellow bow tie twice as often as the red bow tie.

Prove that I didn't see mine, so apart from myself, boys are twice as many as girls.

Suppose there are x girls, X+ 1 boy.

(X+ 1)/(X- 1)=2

(X+ 1)/(X- 1)=4/2

Cross multiplication:

4X-4=2X+2X

X=3

So there are three girls, 3+ 1=4 boys.