10) In a class, a math test was held and the average score was 7. The average score of the boys was 6.5, while that of the girls was 8. If there are 10 boys in the class, how many girls are there?
(A) 4,
(B) 5,
(C) 7
(D) 9,
(E) 11.
10. The answer is $\mathbf{( B )}$.

Let $F$ be the number of females in the class. The average grade of the class is given by the sum of all grades divided by the number of all students in the class. The sum of the grades of the males is equal to their number multiplied by their average grade: $10 \cdot 6.5=65$. Similarly, the sum of the grades of the females is $8 F$. Therefore, we can write:

$$
\text { average grade of the class }=\frac{65+8 F}{10+F}=7 \text {. }
$$

From which we derive $F=5$.

[Problem proposed by P. Negrini.]