Cubes over the terminating denominators

Let SS be the set of positive integers kk for which 1k\tfrac1k has a terminating decimal expansion (e.g. 2∈S2 \in S since 12=0.5\tfrac12 = 0.5, but 7∉S7 \notin S). Evaluate ∑k∈S1k3\displaystyle\sum_{k \in S} \frac{1}{k^3}.

Show hints (2)+
  1. 1k\tfrac1k terminates   ⟺  \iff k=2a5bk=2^a5^b. Factor the sum over aa and bb separately.
  2. (∑8−a)(∑125−b)=87⋅125124=250217\big(\sum 8^{-a}\big)\big(\sum 125^{-b}\big)=\tfrac87\cdot\tfrac{125}{124}=\tfrac{250}{217}.

Answer

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1.1521 (± 0.005)

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