Find the invalid no.from the following series 3, 8, 15, 31, 63…

Find the invalid no.from the following series 3, 8, 15, 31, 63, 127, 255

This problem repeats another one in the library (at most with different numbers), so it's left out of browsing.

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AQuA-RAT — Algebra Question Answering with Rationales (Ling et al., DeepMind, 2017) · train #7869

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