What number would replace the question mark in the series below? 1.5, 4, 7.5, 12, 17.5, 24, 31.5, ?

The next term is 40.

The differences are 2.5, 3.5, 4.5, 5.5, 6.5, 7.5.

Each difference rises by a fixed 1.

The next difference is 7.5 + 1 = 8.5.

31.5 + 8.5 = 40.

The differences form an arithmetic progression.