2.1. At the beginning of January 2020, Borya and Vitya started saving money for a new phone. On the 20th of each month, Borya saved 200 rubles, and Vitya saved 300 rubles. One day, Borya got tired of saving money and stopped (but he did not spend the money he had already saved). Vitya continued to save. On May 11, 2021, Vitya's savings were 6 times more than Borya's. In which month did Borya stop saving? Specify the month and year.
Answer: May, 2020

Solution. By May 1, 2021, Vitya will have been saving for $12+4=16$ months, by which time he will have accumulated $16 \cdot 300=4800$ rubles. If at this point his savings are 6 times more than Borya's, then Borya will have saved $4800: 6=800$ rubles. He will save such an amount over $800: 200=4$ months, so in April he will still be saving money, and in May he will stop.