Sum row until criteria is met (driving me insane!)

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

Sorry yes your assumptions are correct. Will try it at work tomorrow. It looks similar to options I have tried and failed with already so if it works ill be both over the moon and more confused!

It wont let me press enter (or ctrl shift enter) on the formula, it highlights the last "COLUMNS" part as if this is the error.

its excel 2003 incase that matters

with some tweaking i got this:

=IF(SUM(AB2:AF2)<(0.8*C2),"Have not reached 80%",INDEX(AB$1:AF$1,MATCH(TRUE,INDEX(SUBTOTAL(9,OFFSET(AB2,,,,COLUMN(INDIRECT("1:"&(AB2:AF2)))))>=(0.8*C2),),0)))

and ctrl shift enter has it working.
I will put it into my full sheet rather than my 1 row test sheet but currently you have earned hero status!!!

ok so it works (almost).

The first week of my array (so AB2) does not have sales units in for every line im looking at. which gives a ref error. I think i need to make the array ranges dynamic. I have another cell that tells me the first week on sale, so will need to use this to start the array, in place of AB2 (and therefore in place of AB1 too).

What i asked for is solved though, i just need to adapt it slightly to suit.

Thank you so much

@ Mbarnes

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