How To Calculate Percentage of a TextBox Value in Asp.Net C#

declare @tot1 decimal(18,2)
set @tot1 = (5.5/100*1000)
select 1000+@tot1

 double tot1;
            double tot2;
            tot1=(5.5/100) * 1000;
            tot2 = 1000 + @tot1;

double TxtValue =  0.0;
double.Tryparse(TextBox1.Text, out TxtValue );
double ResultValueL = TxtValue  + (TxtValue * 5.5/100)

        protected void Page_Load(object sender, EventArgs e)
        {
            Label1.Visible = false;
        }

        protected void btn_Click(object sender, EventArgs e)
        {
            decimal a =decimal.Parse(TextBox1.Text);
            decimal b = decimal.Parse(TextBox2.Text);
            decimal c = a * b/100;
            decimal d = a + c;
           string s= String.Format("{0:0.00}", d);
            Label1.Text = s.ToString();
            Label1.Visible = true;
        }

    var a = 1000;// Input
    var b = (1000 * 5.5) / 100;
    var c = a + b;
    Console.WriteLine(c);

int a= Convert.ToInt32(txt1.Text);// i.e. 1000
int b= Convert.ToInt32((Convert.ToFloat(txt2.Text)/100))*a;
int c=a+b;

//code behind
float percentage;
percentage = ((float)(gradeA/students))*100.0);
percentLabel.Text = String.Format("{0.00}", percentage);
//using javascript
< script language="javascript">
function calculatePercentage()
{
    var szPercentage = "5.5";
     var result = (szPercentage / 100) * 1000
    result = result + 1000;
}
< /script>