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?
The last marble will be ‘white’. 🙂
Since marbles can only be taken out in pairs and you started off with an odd number of whites there is always going to be one white left over that you’ll keep putting back in the box until it’s left on its own.