In given series 1, 2, 6, 15, 31, 56, 91 only one…

In given series 1, 2, 6, 15, 31, 56, 91 only one number is incorrect. Find out that wrong number

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

AQuA-RAT — Algebra Question Answering with Rationales (Ling et al., DeepMind, 2017) · train #57006