How can I change the default cart behavior?

Here is my site Read-Only: [LINK][1]

This is the demo URL: Shop
So Just look at the cart options. When you add a product the cart option will appear from the right panel. So when you increase or decrease the weekly counter, the total weeks will show at the bottom where it says total weeks. now it’s showing the price. So how can I calculate the weeks in real time to change the default price calculator ?