3. Find all three-digit numbers that decrease by 5 times after the first digit is erased.
Solution. Let $n=abc$ be the desired number. According to the condition, $100a + 10b + c = 5(10b + c)$, which means $25a = 10b + c$, i.e., $c$ is divisible by 5. If $c=0$, then $5a = 2b$, so $b=5, a=2$ (if $b=0 \Rightarrow a=0$). If $c=5$, then $5a = 2b + 1$, so $b=2$ or $b=7$.

Answer. $125, 250, 375$.