284. Two-digit numbers from 19 to 80 are written in sequence. Is the resulting number 192021 ,.. ... 7980 divisible by 1980?
284. Answer: divisible. The given number is obviously divisible by 20. Divisibility by 99 follows from the fact that $100=99+1$, and the equality $19+20+\ldots+80=99 \cdot 31$.