10. Between 200 and 300 children enrolled in the first grade of school. It was decided to form classes of 25 students, but it turned out that ten would not have a place. Then they formed classes of 30 students, but in one of the classes, there were 15 fewer students. How many children enrolled in the first grade?
Solution: $\left\{\begin{array}{ll}a=25 R+10=25(R+1)-15, & a+15: 25, \\ a=30 l-15, & a+15: 30,\end{array}\right\} \Rightarrow a+15 \vdots 150$. Answer: $a=285$.

$$
a+15=150, \underline{300,} 450
$$