Find the number that can replace the question mark (?) in the series given below: 1.5, 5, 10.5, 18, 27.5, 39, ?

The next term is 52.5.

Differences run 3.5, 5.5, 7.5, 9.5, 11.5, rising by 2 each time.

The next difference is 11.5 + 2 = 13.5.

Add it to the last term: 39 + 13.5 = 52.5.

So the missing number is 52.5.