Then according to the meaning of the question "Every boy sees as many white helmets as red helmets", one of the boys is an observer, and he can't see the white hats on his head, so the result he sees is-1 is the number of red hats, and the first equation is obtained: x-1= y;
The title also says that "every girl sees a white helmet twice as often as a red helmet". Similarly, as an observer, the girl can't see the red hat on her head, so the result she sees is: the number of white hats =2× (red hats-1), and the second equation is obtained: x=2(y- 1).
Two equations make up a system of equations.
Solution: x=4, y=3.
That is, there are four boys and three girls.
Then this group of students is 4+3=7.