Arithmetic equation in product info?

[Guest]

I need my shop to calculate travel expenses for me over 40 miles. In this product - http://julietta.co.uk/main/product_info.php?cPath=1&products_id=31 - I want to have the following equation:

 

(field) - 40 * 0.8 = ?

 

Where (field) is a text box where the customer enters a value already obtained from the postcode distance calculator. I then want it to automatically calculate the answer and put that in the shopping cart. Is this remotely doable?

[Guest]

Anyone?