Blue or red?

There are 13 blue balls and 15 red balls in a box. Outside the box, there also 28 red balls.

Remove two balls randomly from the box. If they have different colours, put the blue one back to the box. If they have same colour, take them out and put a red ball in the box. Repeat this until only one ball remains in the box. What colour of the last ball in the box?

Solution →
(better not open this if you don’t have any answers yet)

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