Newbie: changing the value of a field on form

Hello...

I am a newbie to Access or any database for that matter. So please be gentle.
I'm in the process of writing an automotive invoicing database and have gotten stuck early in the game.
On my form I have Qty1 - Mat1 - and Amt1 fields placed from my database.
In Amt1 I would like to typein an amount and have it multiplied by 15% (.015).
I fiddled with the "On Enter" in the Amt1 property box but got nowhere.
Can someone help???

Thanks for your time
d9085@hotmail.com

For those of you that read this and are confused as to solving this problem I was fortunate enough to get some help.
Here is how it was solved:
Using the Code Builder on the After Update line in the properties box for Amt1 I type:
Amt1=Amt1+(Amt1*.15)

Hope this helps someone else