JSFiddle - React, Tailwind, and code Playground

HTML

Inputs:<br>
<input id="boxAmount1" class="amountbox" type="number"><br>
<input id="boxAmount2" class="amountbox" type="number"><br>
<input id="boxAmount3" class="amountbox" type="number"><br>
<input id="boxAmount4" class="amountbox" type="number"><br>
<input id="boxAmount5" class="amountbox" type="number">
<hr>
Grand total:<br>
<input id="boxGrandTotal" type="text">

JavaScript

$(document).on('blur', '.amountbox', function() {
  var grandtotal = 0;
  $('.amountbox').each(function() {
    var v= +$(this).val();
    if(v>'') {
      $(this).val(v.toFixed(2));
      grandtotal+= v;
    }
  });
  $('#boxGrandTotal').val(grandtotal.toFixed(2));
});

$('.amountbox:first').focus();