Generating average monthly values based on weekly values

Hi everybody,

I've run into a bit of trouble with some excel work so I thought id post here to see if u would be able to help me . Basically what I am trying to do atm is generate a formula that takes all weekly prices, and in turn generates monthly prices off them automatically.

The problem I run into is due to the irregularity in number of weeks in a month, I cant create a formula that averages at a regular interval. Rather, it somehow has to group averages based on the number of a the month of each week. I've included a picture of more or less what i am working with (unfortunately i cant disclose actual price values)

Any help would be greatly appreciated

rgrds,
Marcus

Formula works! thank you very much for the help. :D

Just another question...
Some of the customers require middle of the month price averages and so have averages drawn from the middle of the month(week of the 15th/16th) to the middle of the next month. Is there a way to alter the formula to work from the middle of the months rather than than their start?

Any ideas?
I've added an excel sheet with the relevant information and added some dummy data in a similar range to what I'm working with.

rgrds,
Marcus