In each series, look for the degree and direction of change between…

In each series, look for the degree and direction of change between the numbers. In other words, do the numbers increase or decrease, and by how much Look at this series: 1.5, 2.3, 3.1, 3.9, ... What number should come next?

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 #13659