Calculate Total Amount without VAT if TextBox is hidden using JavaScript

Country: Bahrain( i am selecting) Amount:1000 Bank charges: 100(i am entering) Vat:5%(entering) result:50(autogenerate) Discount:6%(entering) result:60(autogenerate) Total:1070(autogenerate)

This is working fine. I am getting the Total(with VAT).

Country:India(i am selecting) Amount:1000 Bank charges: 100(i am entering) Discount:5%(entering) result:50(autogenerate) Total:(here i am not getting the amount) Here i am hiding the field VAT by using attr.Visible=false. What should i do in order to sort out this issue?

function calculate(amount) {
    var bankcharge = document.getElementById("<%=txtBankCharge.ClientID%>").value;
    var vat = document.getElementById("<%=txtvatt.ClientID%>").value;
  var discount = document.getElementById("<%=txtDiscount.ClientID%>").value;
               
    var sum = 0;
    
    $(".amt").each(function () {
         
        if  (isNaN(this.value))
        {
            alert("Please enter numbers only");
            return false;
        }
        
        if (!isNaN(this.value) && this.value.length != 0) {
            sum += parseFloat(this.value);
 
        }
         
    });
         
    
    if (bankcharge.length > 0) {
        if (isNaN(bankcharge)) {
            alert("Bank charge should be numbers only !!");
            return false;
        }
        else {
            sum = sum + parseFloat(bankcharge);
        }
    }
 
    if (vat.length > 0)
    {
        if (isNaN(vat)) {
            alert("VAT should be numbers only !!");
            return false;
        }
        else {
            sum = sum + parseFloat(vat);
        }
    }
 
 
    if (discount.length > 0) {
        if (isNaN(discount)) {
            alert("Discount amount should be numbers only !!");
            return false;
        }
        else {
            sum = sum - parseFloat(discount);
        }
    }
    var atemp = sum.toString().split(".");
    if (atemp.length > 1) {
        sum = sum.toFixed(2);
    }
     document.getElementById("<%=txtTotal.ClientID%>").value = sum;
    document.getElementById("<%=txtAmountInWords.ClientID%>").value = convertNumberToWords(sum);
}
 
 
 
function vatCalc()//added by chetan
{
    var sum = 0;
 
    $(".amt").each(function () {
 
        if (isNaN(this.value)) {
            alert("Please enter numbers only");
            return false;
        }
 
        if (!isNaN(this.value) && this.value.length != 0) {
            sum += parseFloat(this.value);
 
        }
 
    });
 
var _txt2 = document.getElementById('<%= txtvat.ClientID %>');
var _txt3 = document.getElementById('<%= txtvatt.ClientID %>');
var t1=0, t2=0;
 
//if(_txt1.value != "") t1=_txt1.value;
if(_txt2.value != "") t2=_txt2.value;
 
_txt3.value = (sum * parseFloat(t2) )/ 100;
      
}
 
function discountCalc()
{
     var sum = 0;
 
    $(".amt").each(function () {
 
        if (isNaN(this.value)) {
            alert("Please enter numbers only");
            return false;
        }
 
        if (!isNaN(this.value) && this.value.length != 0) {
            sum += parseFloat(this.value);
 
        }
 
    });
 
var _txt2 = document.getElementById('<%= txtDiscountText.ClientID %>');
var _txt3 = document.getElementById('<%= txtDiscount.ClientID %>');
var t1=0, t2=0;
 
//if(_txt1.value != "") t1=_txt1.value;
if(_txt2.value != "") t2=_txt2.value;
 
_txt3.value = (sum * parseFloat(t2)) / 100;
 
}