Try this:
DECLARE @tmp TABLE([name] VARCHAR(30), [qty] INT, [open qty] INT NULL)
INSERT INTO @tmp ([name], [qty], [open qty])
SELECT 'aaa', 1200, 1500
UNION ALL SELECT 'bbb', 2000, 5000
UNION ALL SELECT 'aaa', 200, 300
SELECT [name], [qty], [open qty], [open qty]- [qty] AS [total]
FROM @tmp
Result:
name qty open q. total
aaa 1200 1500 300
bbb 2000 5000 3000
aaa 200 300 100