Hi all, I feel a little sheepish posting this question, but I really can't
get this to work:

I have a list of customer accounts in Sheet 1 column A, with sales turnover
in column C. Some of those accounts belong to the same group of companies. I
have the list of account numbers in, say Sheet 2 A1:A10.

How do I sum the sales turnover of the accounts in Sheet 1 that appear on
the list of account numbers in sheet 2? I've tried sumif and sum if as array
functions, but I can't quite find the right formula to use.

Thanks.