In a box you have 13 white marbles and 15 black marbles.
You also have 28 black marbles outside the box. Remove 2 marbles, randomly, from the box.
If the marbles are of different colours, put the white one back in the box. If they are of the same color, remove both of them and replace them with a black marble from outside the box.
Continue this process until only 1 marble remains in the box.
What colour is the last marble?