Any Add a Number Pad or Calculator will get you and I'm certain that anyone that's actually read and studied Corinda & Annemann already knows at least a couple of methods for doing this (funny thing books, people buy them and look at them for a day and then forget how to open them back up again in order to RESEARCH things and answer questions they already have the answers to . . . not pointing fingers as the original poster, just a general observation).
You can likewise go to OUTLAW Magic and get a copy of Bill Cushman's THE FOURCE.... it's an awesome twist on an old idea that many of us fell in love with a few years back. I actually use it in a couple of different routine these days, primarilly in my Bell, Book & Candle Seance.