function calculate() {
 var nInvoice = parseFloat(document.loan.invoice.value);
 var nDown = parseFloat(document.loan.down.value);
 var nP = document.loan.province.selectedIndex;
 var nProvtax = parseFloat(document.loan.province[nP].value);
 var nN= document.loan.length.selectedIndex;
 var nTerm = parseFloat(document.loan.length[nN].value);
 var nRate = parseFloat(document.loan.rate.value); 
 var nTrade = parseFloat(document.loan.trade.value);
 var nProcentage = parseFloat(document.loan.procentage[nProvtax]);
 var nGst = 0;
 var nPst = 0;
 
 
 if (nP == 0) {
   nGst = ((nInvoice - nTrade)/100)* 5;
   nPst = (((nInvoice - nTrade)+ nGst)/100) * nProvtax;
 }
 else if (nP == 1) {
  nGst = 0;
  nPst = ((nInvoice - nTrade)/100) * nProvtax;
 }
 else if (nP == 2) {
   nGst = 0;
   nPst = ((nInvoice - nTrade)/100) * nProvtax;
 }
  else if (nP ==3) {
   nGst = 0;
   nPst = ((nInvoice - nTrade)/100) * nProvtax;
   name="H.S.T.";
 }
 else if (nP == 4) {
  nGst = ((nInvoice - nTrade)/100) * 5;
  nPst = (((nInvoice - nTrade) + nGst) / 100) * nProvtax;
 
 }
 else if (nP == 7) {
  nGst = ((nInvoice - nTrade)/ 100) * 5;
  nPst = (nInvoice / 100) * nProvtax;
  }
  else if (nP == 13){
  nGst = 0;
  }
 else  {
  nGst = ((nInvoice - nTrade ) / 100) * 5;
  nPst = ((nInvoice - nTrade) /100) * nProvtax;
 } 
 
 var nFinanced =  (nInvoice  + nGst +nPst) - (nTrade + nDown);

  var nI = nRate / 100 / 12;

  var nY = Math.pow(1 + nI, nTerm);

 
  var nPayment = (nFinanced * nY * nI) / (nY -1);
  var nInterest  = (nPayment * nTerm) - nFinanced;
  
  
  document.loan.interest.value =Math.round(nInterest);
  document.loan.payment.value=Math.round(nPayment);
  document.loan.financed.value=Math.round(nFinanced);
  document.loan.Gst.value=Math.round(nGst);
  document.loan.Pst.value=Math.round(nPst);
  document.loan.procentage.value=Math.round(nProvtax)+5;
 }