How to add values of columns of different tables of dataset

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.NET

Respected Sir..
I have 4 datasets

Dim ds_Purchase As New DataSet
Dim ds_Sale As New DataSet
Dim ds_OpeningStock As New DataSet
Dim ds_ClosingStock As New DataSet
Dim ds_Final As New DataSet

each dataset having table with same column
ds_Purchase.table(0)

SID   ItemName  Size    Unit      Qnty
---------------------------------------
120     abc        10      kg        10
130     pqr        20      kg        20
140     lmn        25      gm        40

ds_Sale.table(0)

SID   ItemName  Size    Unit      Qnty
--------------------------------------- 
120     abc        10      kg        10
130     pqr        20      kg        50
150     HIJ        150     gm        60

likewise for all datasets
i want union of all dataset's table with addition of respective quantity into new dataset table
ds_Final.table(0)
ex.

ds_Final.table(0)
SID   ItemName  Size    Unit      Qnty
---------------------------------------
120     abc        10      kg       20 (10+10)
130     pqr        20      kg       70 (20+50)
140     lmn        25      gm       40 (0+40) 
150     HIJ        150     gm       60 (0+60)

I have tried but its takes lots of time to execute since i have to check each and every rows to add quantity
Can we do it by another way ?
Please Help me..
Thank you.