Counting number of cells until a criteria is met
 

I have googled and searched for ever on this and all the solutions i have found do not work on my example, so please help.

C / G / AA / AB / AC / AD / AE /
buy units / answer / week1 / week2 / week3 / week4 / week5
10,000 / xxx / 2,000 / 3,000 / 2,000 / 2,000 / 3,000/


Table above is a very simple example of what i have. I would like a formula that will tell me which week i get to 80% of my buy units total.
So it needs to sum up until it gets to 8,000 basically then tell me the column reference or week reference or whatever.

I found a formula that worked if info was in columns not rows.
I found a formula that worked as long as the answer wasnt required in the same row as the data was.

I have tried MMULT, MATCH etc etc to no avail so far.

It is driving me crazy!

ps sorry for the poor formatting of the example, hope its clear