public ActionResult Create()
        {
            return View();
        } 

        //
        // POST: /Home/Create

        [HttpPost]
        public ActionResult Create(Person person)
        {
            person.Tempo3 = db.persons.Sum(o => o.Tempo1 * o.Tempo2);
            if (ModelState.IsValid)
            {
                
                db.persons.Add(person);
                db.SaveChanges();
                return RedirectToAction("Index");  
            }

            return View(person);

@using (Html.BeginForm()) {
@Html.EditorFor(model => model.Tempo1)
@Html.EditorFor(model => model.Tempo2)
@Html.HiddenFor(model => model.Tempo3)
}