Who's up for a chalenge

[Monzer]

I have a feild "Amount" in my Customer tbl;
Now in the customer EditForm i have a buttom called BackCom for commisions; the buttom opens up another form with the "BackCom" feild for input and the value inserted in this feild will be calculated as folow: ((value*10%)+"Amount&quot
"Amount" is the commision that i get payed on in my main table.
Every week i get stats for my clients concerning their payments,a client is alowed for weekly payments instead one full payment; now,i get 10% comission each time a payment is made rgardless...
Example: CustomerA payed 100$ in week1 ; "Amount"=10$
then CustomerA payed 100$ in week2 ; "Amount"=10$+10$
=20$
The feild "Amount" is increminting all the time

 

[TLady]

So, what was the question ?

 

[Monzer]

Question:How can i do it?
When ever i enter a value the code should calculate it and add it to the "Amount" feild in my customertbl