2. Twenty threes are written in a row on the board. By placing some plus signs between them, Vasya found that the sum is 600. How many plus signs did Vasya place?
Answer: 9 pluses.

Solution. Since there are twenty digits, the number of addends can range from 1 to 20. Divide the sum and all addends by 3. Now the addends have the form $1, 11, 111$, and so on, and the sum is 200. For the sum to end in the digit 0, the number of addends must be divisible by 10. Therefore, the number of addends is either 10 or 20. If there are 20 addends, then all addends consist of a single digit, and the sum is too small. Therefore, there are exactly 10 addends, and the number of pluses between them is 9.

Example of an equation with nine pluses: $333 + 33 \cdot 8 + 3 = 600$.

Criteria. Only the answer - 0 points. Correct example - 3 points. Proven that there are no other options - 4 points. Complete solution - 7 points.