hi .......
i am new in sharepoint

i have three list
1)item master
2)product master
3)product manufacture record

in product manufacture record i have column name product to manufature which will come from product master and another column item required for manufaturing te product which will come from item master..and i have another field number of item require for manufaturing the product it will manually fill.
when ever i fill the value the number of item should be reduce from item maste where i have a field total item .

E.X

number of item require for manufaturing the product =400
it should be subtracted from item Master i.e Total amount
suppose total amount is 1000 after subtraction it should show 600



THANKS in advance if any body help