How Would I: Calculator Page/Form?

I have an existing Drupal 5 site that I would like to add some financial calculation tools to, things like inflation, etc. So what I'd expect to have is a bunch of different pages (each with its own menu entry in navigation) with things like:

Enter Starting Amount: [textbox]
Enter Inflation Rate: [textbox]
Enter Year: [textbox]
Future Value: [blank textbox]
[Calculate Button]

Ideally the user would enter something like $100 at 3% inflation in 2008 and when the [calculate button] is pressed, the math would be done and the [Future Value] textbox would be filled in with "$97.00". These are just one-time calculations and won't need to be stored anywhere.

How would I do this in Drupal?

The only web programming I've done has been in basic PHP, in which case clicking the "calculate" button would load a new page displaying the value, which I could do easily enough. That's obviously not a smooth way to do things in Drupal because you'd just get a new page displaying the answer(s) without the pretty Drupal framework around it.

Forms API? AJAX? PHP? JavaScript? CCK? Contemplate? Something else entirely?

Please forgive my cluelessness -- although my application programming experience could be measured in decades, my web programming experience is only in months. Just point me in the right direction and I'm sure I can work things out...