how to make a flash total calculator

I am making a website for someone entirely on flash. She wants a page that has a total calculator so when someone types in the amount of money they're paying her, then gst and pst get automatically added on to the total, plus another percentage. so for example

service charge - - - - - typed in or choses from a list of rates
gst - - - - - - - - - - - -populates based on the service charge
pst - - - - - - - - - - - -populates based on the service charge
transaction fee - - - - -based on the service charge and gst and pst
---------------
total - - - - - - - - - - -gives the total

anybody who knows about this and can help would be great, thanks for your time.