JSFiddle - React, Tailwind, and code Playground

HTML

<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css">
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/js/bootstrap.min.js"></script>
<div class="form-group">
	<label for="brutto">Brutto</label>
	<input class="form-control" readonly="readonly" name="brutto" type="text" id="brutto">
</div>
<div class="form-group">
	<label for="tax">MWst. 19%</label>
	<input class="form-control" readonly="readonly" name="tax" type="text" id="tax">
</div>
<div class="form-group">
	<label for="netto">Netto</label>
	<input class="form-control" name="netto" type="text" id="netto">
</div>

JavaScript

// calculate brutto and tax
$('#netto').on('keyup paste', function () {

  var netto = parseFloat($("#netto").val());
  $("#brutto").val((netto * 1.19).toFixed(2));
  var brutto =  parseFloat($("#brutto").val());
  $("#tax").val((brutto - netto).toFixed(2));

});