Add a script on my web page

Add a script that multiplies the price and the number of day on my web site... On the homepage is a script which allowes me to pick a car type class and transmission type, and when i push "pretrazi" i directs me on the second page where is the list of cars that match with the selected parameters... now i added another droplist, "broj dana" numer of days, and i need that number of days changes the price on the second page... Something like, if i put the number of days 5, that the price on the second page multiplies the number of days with the price that is for that car...