1. On the Island of Knights and Liars, knights always tell the truth, while liars always lie. In a school on this island, both knights and liars study in the same class. One day, the teacher asked four children: Anu, Banu, Vanu, and Danu, who among them had completed their homework. They answered:

- Anu: Banu, Vanu, and Danu completed the homework.
- Banu: Anu, Vanu, and Danu did not complete the homework.
- Vanu: Don't believe them, sir! Anu and Banu are liars!
- Danu: No, sir, Anu, Banu, and Vanu are knights!

How many knights are among these children?
Answer: 1.

Solution: If Vanu is a knight, then all the others are liars.

Let Vanu be a liar. Then Danu is also a liar (since he says Vanu is a knight). And at least one of Anu and Banu must be a knight. Both of them cannot be knights, as they contradict each other. In any case, only one of the children is a knight.