There are three boxes labelled as Red Balls, Black Balls and Red and Black Balls. Each one of the box is labelled incorrectly. You have just one chance to pick up from any box and then label the boxes correctly.
How will you do it?
Pick up from the box that is labelled as Red and Black Balls. Since they are labelled incorrectly, it should have either Red Balls only or Black Balls only.
Suppose you get a Red Ball, then you will be sure that this box has Red Balls only and you can label it that. Now the box that has been labelled as Black cannot have Black balls as all were labelled incorrectly. Also, we know that we have found the Red Balls box. Thus it must be the box with Red and Black Balls. Now the only box that remains is the one labelled as Red Balls. You can label it as Black Balls now.