Hi there,
If I understand you correctly, what you want to do is add up the numbers which a user gives you ?? Well you can do this using javascript if you wish.
You should create the fields where the user will enter their data and then give thiese fields names etc etc.
Then go to the javascript forum here and show us your code and what you want to do and we will point you in the right direction or give you some help.
Hope it helps get you started.