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.