How do I interpolate values into a range?

[hello]

Hi

I would like to interpolate values into a range.

Column B are the data. I would like to linearly interpolate the values in
column B into the ranges in column A. If it is easier, it is OK to assume
that the value in column B is for the midpoint of the ranges in column A.

Thank you
A B
18-19 17.5
20-24 33.8
25-29 53.1
30-34 68.0
35-39 85.3
40-44 97.5
45-49 100.0
50-54 97.2
55-59 91.1
60-64 53.5