Simple Addition Form

I have a form with several text variable fields (30) in which the client is asked to select and assign a weight to each variables that influenced their decision to do something. The total weight of all selected variables must total 100. I have a field "total weight" that I want the sum of the assigned weights to appear. As the client assigns each weight the total weight would update using an onChange(or similar action). I do not want the client to have to click a submit or calculate button in order to have the total displayed. I want to remain on the same form so that the client can make alterations to his/her assigned weights until they equal 100. My web hosting/server provider does not allow any type of sql databasing on their server so I will be gathering the data in a text file so there is not interactivity with a database. Is this such a difficult thing to do, or am I just not seeing the forest for the trees? I'm befuddled. I've even resorted to learning some java and cgi scripting to accomplish this but, I now realize, given my time frame, I don't have time to teach myself all I need to know about these languages to make this work. You can view a sample of the web form I have created thus far at http://www.kingwoodgroup.com/PHPTestExample . I appreciate any direction you can give.