Let's say for sanity sake that you have this table:

pos:
po - number
item - number
qty - number

create query to get sums (called sums)
select
 item,
 sum(qty) as sum_qty
from
 po
group by
 item

create a new query for your final result:
select
 po,
 item,
 qty,
 sum_qty
from
 po
 inner join sums
   on po.item = sums.item

Cheers,
Jason Lepack

SELECT [Item #], Sum(qty) As Qsum
FROM thetable
GROUP BY [Item #]

will produce:
100        5   
200        7

If you really need the result you posted, then Join that
query back to the table:

SELECT T.PO, T.[Item #], T.qty, Qsum
FROM thetable As T
    INNER JOIN (SELECT X.[Item #], Sum(X.qty) As Qsum
                                FROM thetable As X
                                GROUP BY X.[Item #])
        ON T.[Item #] = X.[Item #]