microsoft.public.access

I am a student doing an Access project for Ict. For this project i needed to
create a database for a business i made up. i chose car retailler. Right
now i am using ACCESS 2007 but have saved it in a 02-03 Access format.

Anyway, so far i have created:

-A table to hold car records , this includes the quantity of each car model
as a field name.
-A table for customer records
-& a table for sales which shows the transacti on between the customer & the
car they have bought.

I want to know how do you update the quantity of a certain car model after a
transaction has been made. For example i have "15" of "Car A" this is shown
in the cars records table,, A customer has bought 2 of these cars (This is
shown in the sales records table). After completing this sale, how do you
create a way to update the quantity number of "Car A" . So this number
representing the quantity should change from "15" to "13".

Thank you for taking time in reading this, i would be greatful if you could
reply to it.