Cuts until one orb remains

Seven orbs, labelled 11 (attached to the ceiling) down to 77, hang in a vertical chain, each joined to its neighbor by a link (66 links total). Each step, a uniformly random remaining link is cut, and every orb below that link falls and shatters. What is the expected number of cuts until only orb 11 remains? Round to two decimal places.

Show hints (2)+
  1. Cutting link jj (from the top) leaves j−1j-1 links - uniform on {0,…,m−1}\{0,\dots,m-1\}.
  2. em=1+1m∑i<meie_m = 1 + \tfrac1m\sum_{i<m} e_i solves to em=Hme_m = H_m; take H6H_6.

Answer

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2.45 (± 0.01)

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