filtering/autofill problem
 

I have a list of customers who will receive a mailer (column A), if they
reply then I enter something in column B against their name. I then want to
allocate a sequential number against each customer who has replied. So those
who have something in column B are to be excluded from the sequential list
(if that makes sense). I have tried filtering on on blanks in col B but then
I can't autofill column C with a number. Help please! thanks