Look at this series: 15, 15, ?, 27, 39, 39,

Look at this series: 15, 15, ?, 27, 39, 39, ... What number should fill the blank?

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 #62687