I have a very large excel file, around 50K rows. In Column (C) I have the person numbers 1,1,1,1,2,3,4,5,.... and in Column(N) I have the payments for this person, so the data looks like
What I want is to sum All payments for one person from column N and set the result in column O, then merge column O.
It looks like you're looking for the built-in excel
=sumif(C:C,"specific person number",N:N)