const double TaxRate = 0.0775;
        static double subtotal;
        static double tax = (TaxRate * subtotal);
        static double OrderTotal = subtotal + tax;

        private void radButtonHamburger_CheckedChanged(object sender, EventArgs e)
        {
            subtotal = 0.0;
            txtSubtotal.Clear();
            chkBox0.Checked = false;
            chkBox1.Checked = false;
            chkBox2.Checked = false;

            lblAddOnItems0.Text = "Lettuce, tomato, and onions";
            lblAddOnItems1.Text = "Ketchup, mustard, an mayo";
            lblAddOnItems2.Text = "French fries";
            grpBoxAddOnItems.Text = "Add-on items($.75/each)";

            /* add the main course */
            subtotal += 6.95;

            /* add the extra items */
            if (chkBox0.Checked)
                subtotal += 0.75;
            if (chkBox1.Checked)
                subtotal += 0.75;
            if (chkBox2.Checked)
                subtotal += 0.75;
        }