build a js calculator on a drupal 7

how to calculate with javascript in webform i have form for travel request in form you have a select option ( adults , childs ) i need to update price when select adults or select childs by javascript in same page to calculate the total cost for the travel if you check this website you will understand what I mean I need to create like this form [login to view URL] select any date and you will redirect to form i need to create the same form