9) In a class, blonde students make up $40 \%$ of the total, while the rest are brunettes. Among all blonde students, $75 \%$ are girls. Knowing that in the class the number of girls is equal to the number of boys, what is the percentage of brunette boys out of the total number of students in the class?
(A) $20 \%$
(B) $25 \%$
(C) $30 \%$
(D) $40 \%$
(E) $50 \%$
9. The answer is $(\mathbf{D})$.

Let $F_{b}$ and $F_{c}$ be the percentages of blonde and brown-haired females, respectively, and $M_{b}$ and $M_{c}$ be the percentages of blonde and brown-haired males, respectively. We know that $F_{b}+M_{b}=40$ and $F_{b}+F_{c}=M_{b}+M_{c}=50$. The blonde females are $75 \%$ of the total blonde students, so the blonde males are $25 \%$ of the total blonde students, thus $F_{b}=3 M_{b}$. Then, from the first of the two equalities written above, we find $M_{b}=10$, and therefore from $M_{b}+M_{c}=50$ we have $M_{c}=40$.

[Problem proposed by S. Monica.]