在visual studio中的ASPX文件中，文件不能正确读取。

Here is my issue. I have an assignment where I have to write a simple ASPX page the lets the users input into 3 text boxes and click a calculate button which will return the result of how many items are requested. Here's what I have. The line is suppose to save the users input into the ID txtPlateQty and in the C# code below, which is a separate file in visual studio 2010, I want to take the users input and convert it into an int for calculations. The issue is in the C# file the "Convert.ToInt32(txtPlateQty.Text); does not recognize the variable "txtPlateQty" as if it's out of scope. This is a assignment for a class and my professor has and identical example and his works while mine does not. If anyone could shed some light onto what is causing this issue I would be very grateful. Here is my issue. I have an assignment where I