Find the number that can replace the question mark in the series given below: 5.5, 11, 18.5, 30, 49.5, 85, ?

The next number is 152.5.

The differences are 5.5, 7.5, 11.5, 19.5, 35.5.

The gaps between these differences are 2, 4, 8, 16, which keep doubling.

So the next gap is 32, making the next difference 35.5 + 32 = 67.5.

85 + 67.5 = 152.5.

Thus the required value is 152.5.