function calculate() {
 var nInvoice = parseFloat(document.loan.invoice.value);
 var nDown = parseFloat(document.loan.down.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 nGst = 0;
 var nPst = 0;
 
 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);
 }