4) On Mars, the Grand Marshal of Martian Education has announced that the next school year will reduce the number of school teachers by $30 \%$ and that those who remain in service will receive a $35\%$ increase in salary. Therefore, the overall expenditure on teachers' salaries will
(A) decrease by $5.5 \%$,
(B) decrease by $5 \%$
(C) increase by $5 \%$
(D) remain unchanged,
(E) increase by $7 \%$.
4. The answer is $(\mathbf{A})$.

Let $N$ be the number of teachers present this year and $S$ be their salary for this year. The total expenditure for this year is therefore $S N$. Next year, there will be $\frac{7}{10} \mathrm{~N}$ teachers and the salary of each of them will be $\frac{135}{100} S$. The total expenditure will then be

$$
\frac{7}{10} N \frac{135}{100} S = S N \frac{945}{1000}
$$

which is 94.5% of this year's expenditure. Therefore, the total expenditure will decrease by 5.5%.