Briefly

The marbles

The marbles

We put thirteen white and fifteen black marbles in a box. We also have twenty-eight black marbles out of the box. We remove two marbles from the box and if they are of different color, we put the white one in the box again. If they are the same color, we put a black marble in the box. We continue to do this until there is only one marble left in the box.

What color is the marble left in the box?

Solution

White marbles can only be taken out of the box in pairs, so since there are thirteen white ones (which is an odd number), the last one will be of that color.