1. There are 28 students in the class. 17 have a cat at home, and 10 have a dog. 5 students have neither a cat nor a dog. How many students have both a cat and a dog?
Answer: 4.

Solution: The number of students with cats or dogs is $28-5=23$. If we add $17+10=27$, it exceeds 23 due to those who have both a cat and a dog, who were counted twice. The number of such students is $27-23=4$.

Comment: Correct solution - 20 points. Solution started, with some progress - 5 points. Solution started, but progress is insignificant - 1 point. Correct answer given without explanation - 1 point.