In the first cell of column B, put 1. In the following cell :
=COUNTA($B$1:B1)+1
and drag down as needed.
Column B should be cleared each time.
HTH
Daniel
Let me make it simpler.
column A stores the exam subjects that I am going to filter. I found that
after applying the data--filter--autofilter

(with some creteria) on column A, a certain subject(say English) is
filtered. Then I try to use autofill in column B to

assign seat number to the students(001, 002, 003,..). However, I cannot fill
in the series. Can anybody help me?