Averaging over varying values

Hey all,

I am trying to average Column 'B' based on ranges defined in Column 'A'. The ranges in 'A' vary in length, so there may be 100 rows of depth 6450, and maybe 50 rows of depth 6550; so I would like to average the corresponding 100 rows of column B and get a value and the corresponding 50 rows of column B and get a value. I have attached a sample work book, with the excel average functions shown as reference, and my lame attempt at a code to acquire this result.

Any help would be appreciated, I thought maybe arrays would be the way to go, but I am not doing something right.

THANKS!