filtering numbers?

Hello
I've been trying to find a formula that I need but am unable to. Can anyone help?

There are two columns A & B. In A there is a sum of positive & negative numbers. B contains an average of positive numbers, present value 20. In a 3rd column I need a formula based on column A that includes all numbers that are the average result of colomn B or lower but excludes ones higher than the column B result.

The manually worked out answer should be 16.60.

Any ideas.

Alex G