math problem using asp

i'm using dreamweaver mx and i'm creating an asp page...and in my page is have this form := paid, balance, new balance and budget approved. can i know how to calculate new balance while balance is get from the access 2000(database) and will minus with paid.[balance(from access)-paid=new balance). for your information new balance will storred in the table balance in access.so that every single time that data is entered, the form balance will looking the previous calculation of the new balance. please someone help me