3. In the first quarter, Masha and Lena together received 23 fives, Svetlana and Masha together received 18 fives, and Svetlana and Lena together received 15 fives. How many fives did each of the girls receive?
Answer. Masha -13, Lena -10, Svetlana -5.

Solution. All three girls together received half of $(23+18+15)=28$ fives. Therefore, Svetlana received $28-23=5$ fives, Masha received $18-5=13$ fives, and Lena received $23-13=10$ fives.

Comment. Correct solution - 20 points. There is a gap in the justification - 15 points. The problem is solved by trial and error, and it is not shown that there are no other options - 10 points. The solution is started, and there is some progress - 5 points. The solution is started, but the progress is insignificant - 1 point. The correct answer is given without explanations - 1 point.