To find the sum of the series 1.5 + 2.5 + 3.5 + 4.5 + ... + 8.5, we can use the formula for the sum of an arithmetic series:

Sn = (n/2)(a + l)

where Sn is the sum of the series, n is the number of terms, a is the first term, and l is the last term.

In this case, the first term (a) is 1.5 and the last term (l) is 8.5. We need to find the number of terms (n).

The common difference (d) between consecutive terms is 1. Since each term is obtained by adding 1 to the previous term.

To find the number of terms, we can use the formula:

l = a + (n - 1)d

Rearranging the formula, we get:

n = (l - a)/d + 1

Substituting the values, we have:

n = (8.5 - 1.5)/1 + 1
n = 7/1 + 1
n = 7 + 1
n = 8

So, there are 8 terms in the series.

Now, we can find the sum using the formula:

Sn = (n/2)(a + l)

Sn = (8/2)(1.5 + 8.5)
Sn = 4(10)
Sn = 40

Therefore, the sum of the series 1.5 + 2.5 + 3.5 + 4.5 + ... + 8.5 is 40.