Can someone help me build a similar online caculator ?

Hi,

Im a high school student in web design i was ask to build this online caculator for i person i do work for.
It is not as detailed as this caculator http://calculator.s3.amazonaws.com/calc5.html
but i need something very similar. If you notice when you enter in a number in the top field it sums it up 
in the bottom field automatically.

Basically what i am asking for is for is when you enter in
A amount in one field it will automatically Multiply by a number in another field 
and give a total by itself without having to press a caculate button 
It would look like this http://ghsglobaltech.guhsd.net/students/2009/web/ftp_01_36/Untitled-1.php
very simple & not very flashy

If you know of any tutorials or are able to piont me in the right direction 
it would be very much appriciated 

Thank You so much for your time and assistance. 

You can email me at

chance.house@yourtcr.com