MasterPhantom

thanks though does not recognize ViewBag, only ViewData
Replacing ViewBag.Number with ViewData ["Number"] does not show an error, but passing a number does not print anything.
Is there another way?

The code shared above has been tested and it works just fine.  Share you code if you need a community code review.  Perhaps you are building a Razor Pages application?

@page
@model RazorDemo.Pages.FormModel
@{
}

<div>
    <form id="form" action="" method="post">
        <div>
            <input name="number" type="text" />
        </div>
        <div>
            <input id="Submit1" type="submit" value="submit" />
        </div>
    </form>
    <div>
        <span>25% is @Model.Number</span>     
    </div>
</div>

    public class FormModel : PageModel
    {
        public void OnGet()
        {
        }

        public void OnPost(decimal number)
        {
            Number = (number * 0.25m);
        }

        [BindProperty]
        public decimal Number { get; set; }
    }

Razor Pages Tutorials

https://docs.microsoft.com/en-us/aspnet/core/tutorials/razor-pages/razor-pages-start?view=aspnetcore-3.1&tabs=visual-studio