Depends on whether you mean in Access or by using VB. I have never found a way in Access by itself to query a record and apply its values to a different record. But if you go into VB, then you can open a recordset and store the value for a record into a variable, do the math you want, then save the result in a subsequent record of your selection.
I remember trying to do this when I was just starting with Access and it was very frustrating not to be able to find a way.