Penalty kicks with a shifting rate

You take 100100 penalty kicks. You make the first and miss the second. From then on, the probability you score any kick equals the fraction of kicks you've made so far (e.g. after 1717 makes in 3030 attempts, the next has probability 1730\tfrac{17}{30}). The probability that you finish with exactly 6666 made can be written as 1M\tfrac1M for an integer MM; find MM.

Show hints (2)+
  1. It's a Pólya urn started with one make and one miss (score = past make-fraction, then reinforce).
  2. With a (1,1)(1,1) start the success count is uniform: total makes is uniform on {1,,99}\{1,\dots,99\}.

Answer

Reveal answer →

99

Want the full step-by-step worked solution? It's part of Premium - along with a worked solution for every question in the bank.

Asked at: Jane Street, Two Sigma

Related questions