2. Anya did not tell Misha how old she is, but she informed him that on each of her birthdays, her mother puts as many coins into the piggy bank as Anya is turning years old. Misha estimated that there are no fewer than 110 but no more than 130 coins in the piggy bank. How old is Anya?
Answer: 15. Solution. Either use the formula for the sum of an arithmetic progression: $110 \leq \frac{1+n}{2} n \leq 130$, or simply calculate the sum "brute force".