I have a horse. Do you know what color it is?
Allan said, "I guess it is not black".
Brian said, "It is either brown, or gray".
Charlie said "I know it is brown".
I said, "At least one of you is right and at least one of you is wrong." What is the color of my horse if the color is one of the above?
If the horse is brown, then everyone is right. This is not the answer.
If the horse is black, then every one is wront. This is not the answer either.
Therefore, the horse if gray. to verify the answer, Allan was right, Brian was right, but Charlie was wrong.