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Colour of the final cherry eaten from two bowls

Question

Bowl A has 22 red and 55 green cherries; bowl B has 66 red and 33 green. Repeat the following until every cherry is gone: if bowl B is not empty, move a randomly chosen cherry from B to A; then eat a randomly chosen cherry from A. What is the probability, rounded to five decimal places, that the very last cherry eaten is red?

Short answer

About 0.552130.55213. Track one cherry at a time. During the 99 transfer rounds bowl A always holds 88 cherries when one is eaten, and the final 77 are eaten in random order. An original A cherry is last with probability (7/8)9/7(7/8)^9/7; a B cherry moved in round kk (uniform on 11–99) with probability (7/8)10−k/7(7/8)^{10-k}/7. Weighting by colour gives 27(7/8)9+23(1−(7/8)9)≈0.55213\tfrac27(7/8)^9+\tfrac23\left(1-(7/8)^9\right)\approx0.55213.

Adapted from QuantQuestions.io; question and answer rewritten by SelfMadeQuant.com.

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