3. Zhenya had 9 cards with numbers from 1 to 9. He lost the card with the number 7. Can the remaining 8 cards be arranged in a row so that any two adjacent cards form a number divisible by 7?
3. Answer: no.

Note that after 5 there must be 6, after $6-3$, after $3-5$, after $5-6$. No other digits can precede these either. Therefore, it is impossible to arrange all these digits in one row.