On an island where knaves never tell the truth and knights always tell the truth, you meet 3 of the island's inhabitants: Fred, George, and Quincy. You ask Fred, "Are you a knave?" Fred says the answer to the other natives, but you can't hear him. George turns to you and says that Fred denied being a knave. Quincy says Fred is a knave.

Answer:

Let's analyze the two possible situations, from the Fred's answer

Situation 1: Fred is a knave.

If Fred is a Knave, he must have told to the other natives that he wasn't, remember that Knaves will never tell the truth. In that sense, George said that Fred denied being a knave, he would be telling the truth (Fred is a knight) and Quincy said that Fred is a knave, which would also be true (Quincy is a knight too).

Situation 2: Fred is a knight

If Fred is a knight, he must have told to the other natives that he wasn't a knave, he would be telling the truth. In this case, George would also be telling the truth by ensuring that Fred denied being a knave (Fred is a knight). However, Quincy would be lying to ensure that Fred is a knave. In this situation Quincy is the knave

Thus, in both situations, there is only one knave.

so, the right answer is one.