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FRM Part I · FRM Exam Part I · Binomial Trees

In a three-step recombining binomial tree where an up move followed by a down move gives the same price as a down move followed by an up move, how many distinct terminal stock prices exist, and how many distinct paths lead to them?

There are 4 distinct terminal prices and 8 paths. A recombining tree gives n+1 terminal nodes after n steps, here 3+1 = 4, based on the number of up moves. The number of paths is 2^3 = 8.

  1. A3 distinct prices, 6 paths
  2. B4 distinct prices, 6 paths
  3. C4 distinct prices, 8 pathsCorrect
  4. D8 distinct prices, 8 paths

Explanation

After n steps a recombining tree has n+1 terminal prices, so three steps give 4 (uuu, uud, udd, ddd). Each step has two choices, so there are 2^3 = 8 paths. Prices depend only on the number of up moves, which is why the paths collapse into 4 prices. The 8-prices option describes a non-recombining tree.

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