IMO Shortlist 1982 problem 10

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Dodao/la: arhiva
April 2, 2012
A box contains p white balls and q black balls. Beside the box there is a pile of black balls. Two balls are taken out of the box. If they have the same color, a black ball from the pile is put into the box. If they have different colors, the white ball is put back into the box. This procedure is repeated until the last two balls are removed from the box and one last ball is put in. What is the probability that this last ball is white?
Source: Međunarodna matematička olimpijada, shortlist 1982