7. (10 points) Boys wear red hats, girls wear yellow hats, and teachers wear blue hats. Each person cannot see their own hat. Xiaoqiang (a boy) sees 2 more red hats than yellow hats, Xiaohua (a girl) sees twice as many yellow hats as blue hats, and the teacher sees 11 fewer blue hats than red hats. Therefore, there are $\qquad$ girls.
【Solution】Solve: Let there be $x$ girls, then there are $x+3(x+2+1=x+3)$ boys, so $(x-1) \div 2=x+3-11+1$
$$
\begin{aligned}
0.5 x-0.5 & =x-7 \\
0.5 x-7 & =-0.5 \\
0.5 x & =6.5 \\
x & =13
\end{aligned}
$$

Answer: There are 13 girls.
Therefore, the answer is: 13.