10. Koschei is counting his gold coins. When he counts them by tens, there are 7 coins left, and he is 3 coins short of a whole number of dozens. Koschei's wealth is estimated at $300-400$ coins. How many coins does Koschei have?
Solution: $\left\{\begin{array}{ll}a=10 x+7=10(x+1)-3, & a+3 \mid 10, \\ a=12 y-3, & a+3 \mid 12,\end{array}\right\} \Rightarrow a+3 \mid 60$. Answer: $a=357$.

$$
a+3=300,360,420
$$

## TICKET № 7