how to sum begining balance

Hi, i'm new in using crystal report 8.5 and SQL 7, now i'm trying to create report begining balance, see below details:
date item qty
12/1/05 001 150
12/12/05 002 200
12/23/05 003 250
1/1/06 001 10
1/12/06 002 15
1/23/06 003 20
2/10/06 001 100
2/15/06 002 150
2/27/06 003 200

how to sum begining balance quantity every month?
i want like these:

jan 06
beg item
150 001
200 002
250 003

feb 06
beg item
160 001
215 002
270 003

million thanks in advancegroup by date , for every month

u will get
dec 05
beg item
150 001
200 002
250 003

suppress dec 05 and put a formula in group header

dateadd('m',-1,suppresseddatefield)

hope it helps|||Sraheem thanks for your reply but sorry is not like i want i'm bit confius to describe, let me try like below:

Item Description Beg In Out End
1614 0.125 X 60 mm 346 131
1671 0.075x60mm 3,196 340
1626 Dia 0.85 mm 13,114 0

i need to create the formula for beg balance.
i have question, is it possible to create sum formula using three condition like sum ({qty},{item},cut off date)
in which to calculate beg balance I must sum qty until cut off date
then after i find beg balance i can calculate end balance.