6) At the end of last year, in a school, $18 \%$ of all the students graduated and another $3 \%$ of the students transferred to other schools. This year, 84 new students enrolled in the school, and now the number of students is equal to that of last year. How many students does the school have?
(A) 324,
(B) 400,
(C) 500
(D) 525,
(E) 600.
6. The answer is $(\mathbf{B})$.

Let $N$ be the number of students in the school, which is the same for this year and last year. At the end of last year, a total of $21\%$ of the students left the school (the sum of those who graduated and those who transferred). This year, 84 new students arrived. Since the total number of students has remained unchanged, we deduce that $21\%$ of $N$ is equal to 84. Therefore, $1\%$ of the students is $84 / 21 = 4$ and $N$ is 400.

[Problem proposed by F. Mugelli.]