7.4. On the board, the number 321321321321 is written. Which digits need to be erased to obtain the largest possible number divisible by 9?
Answer: erase the last two threes.

Solution: From the divisibility rule for 9, it follows that the sum of the erased digits must be equal to 6. Since the larger the number, the more digits it has, we need to erase two threes. This will leave a number with 10 digits. To make this number as large as possible, we need to have larger digits in the higher place values, so we erase the last two threes.