2.3. On January 11, 2011, Baba Yaga and Koschei the Deathless decided to compete in who was kinder. Each month, Baba Yaga did good deeds for 77 children, while Koschei the Deathless did so for 12. But one day, Koschei got tired of being kind. On January 11, 2021, Baba Yaga calculated that the number of her good deeds had become 5 times greater than Koschei's (and she also stopped doing this). In which month did Koschei stop doing good deeds? Specify the month and year.
Answer: March, 2012

Solution. Baba Yaga did good for 10 years, which is 120 months in total. Therefore, Baba Yaga did 7 $\cdot$ 120 $=840$ good deeds, and Koschei did $840: 5=168$ deeds. $168: 12=14$, so Koschei seemed good for 14 months, and stopped being good on the fifteenth month.